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Roma may have reached the semi-finals of last season's tournament but a repeat showing looks highly unlikely right now after picking up just nine points in a very soft group.
Indeed, even though Real Madrid have been mediocre at best, the Giallorossi were beaten home and away by the Blancos, conceding five unanswered goals in the process.
Roma only progressed by winning three of their four games against minnows CSKA Moscow and Viktoria Plzen but the final-round loss to the latter only served to ramp up the pressure on coach Eusebio di Francesco and his maddeningly inconsistent side, who are struggling in Serie A.
In Edin Dzeko, the Italians still possess one of the most effective no.9s in world football, while teenager midfielder Nicolo Zaniolo looks a major find, but Roma are weak at the back, a problem exacerbated by the loss of the world-class Brazilian goalkeeper Alisson and his subsequent replacement with the sub-standard Robin Olsen.
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#15 Manchester United
So far have standards fallen at Old Trafford, Manchester United probably deserve credit for finishing as runners-up in a group containing Juventus, Valencia and Young Boys.
However, the unimpressive manner of their qualification suggests Jose Mourinho's men have next-to-no chance of actually winning the title, given the manager still doesn't know his best centre-half pairing, has fallen out with his best midfielder, Paul Pogba, and has only Marcus Rashford firing in attack.
Indeed, things were looking grim after their loss at home to Juventus on matchday three but the unlikeliest of late comebacks after being dominated in Turin turned their entire campaign around, while they were also indebted to Marouane Fellaini for an injury-time winner in their crucial clash with Young Boys.
Still, the error-prone performance in defeat at Mestalla – summed up by Phil Jones' own goal – underlined that Mourinho is a man with many problems to address between now and February... if he lasts that long, of course.
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A sold but unspectacular side, Schalke reached the last 16 by scoring just six goals in six games. Crucially, though, they only conceded four, and three of those came in their one defeat, in Porto.
There is talent at the Veltins-Arena. Youngsters Breel Embolo and Weston McKennie are players of real potential, Yevhen Konoplyanka is a very dangerous opponent on his day, while Matija Nastasic was one of the best centre-halves on show in the group stage.
However, Schalke's domestic form – they are currently 13th in the Bundesliga – suggests that Domenico Tedesco lacks the strength in depth to take his team far into the competition.
Indeed, given they cannot be paired again with Porto or another German side, there will be no easy draw in the last 16 for the Gelsenkirchen outfit, who will face either Barcelona, Manchester City, Juventus, Paris Saint-Germain or Real Madrid.
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What a bizarre group-stage campaign for Lyon, who didn't lose a single game yet won just one.
The French side drew five times in total and only progressed thanks to Nabil Fekir's fine, second-half equaliser away to Shakhtar Donetsk on matchday six.
Still, nobody would begrudge Lyon their place in the last 16, given they took four points off favourites Manchester City.
Indeed, after two thrillingly bold and adventurous outings against the Premier League champions, they were only denied a maximum haul by Sergio Aguero's late leveller in France.
Lyon aren't among the favourites but whoever draws Bruno Genesio's side will be rightly wary of a team containing talent such as France international Fekir, a resurgent Memphis Depay, the exciting and unpredictable Moussa Dembele and colossal midfielder Tanguy Ndombele.
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Porto made short work of arguably the weakest but most evenly balanced group in this season's competition, winning all but one of their matches against Schalke, Galatasaray and Lokomotiv Moscow.
Indeed, after coming from behind to claim a 1-1 draw in Gelsenkirch on matchday one, the Primeira Liga leaders reeled off five victories on the bounce to qualify with a tournament-high 16 points.
Porto may well be the team that every unseeded side wants to face in the last 16 but they should not be underestimated.
They boast a proven Champions League winner in goalkeeper Iker Casillas, are led by the excellent Mexico midfielder Hector Herrera and Moussa Marega is in fine goalscoring form, having netted five times already.
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Manager Mauricio Pochettino felt his side had pulled off "Mission: Impossible" in qualifying for the last 16, and it's easy to understand why.
After suffering a heart-breaking and undeserved opening-round loss at Inter, struggling Spurs found themselves with just one point from three games after being on the receiving end of a Lionel Messi masterclass at Wembley and then being held to a disappointing draw at PSV.
However, they proceeded to progress with just eight points, and a negative goal difference, by netting crucial late goals to earn a win over PSV, defeat Inter at home and then snatch a draw at Barcelona.
Of course, the Nerazzurri's surprising inability to beat PSV on matchday six proved decisive but Tottenham are an undoubtedly superior squad, one blessed with a plethora of exciting attacking talents, such as Harry Kane, Christian Eriksen, Dele Alli, Lucas Moura and Son Heung-min, and no group winner will want to face them in the last 16.
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#10 Real Madrid
Real Madrid may have progressed to the last 16 as group winners but, at the moment, they look anything but potential outright winners.
The Blancos have undoubtedly improved since Santiago Solari replaced the hapless Julen Lopetegui as coach midway through their group-stage campaign but the defending champions did not play with anything like their usual swagger.
Even their most noteworthy victory, a 3-0 defeat of Roma at the Bernabeu, came at a time when their opponents were in total disarray and they were fortunate to win the return leg 2-0, with the Italians having squandered a succession of clear-cut chances during a dominant first-half display at the Stadio Olimpico.
The 1-0 loss to CSKA Moscow was dismal and even though Solari rested several key men for the return match in Madrid, the historically bad 3-0 defeat, coupled with their stuttering Liga form, merely serves to underline that Real are more vulnerable than at any point since Rafa Benitez's time in charge.
It is difficult to know what to make of Juventus, who have become even more dominant domestically thanks to the summer signing of Cristiano Ronaldo, yet remain prone to costly lapses in concentration in Europe.
Of course, the Bianconeri won their group but not in the fashion that was expected, given the low quality of the opposition: Manchester United, Valencia and Young Boys.
There were impressive victories at Mestalla and Old Trafford as Juve reached the halfway point with a maximum haul of nine points but cracks have appeared since the shocking late capitulation at home to United.
Indeed, in the end, Massimiliano Allegri's men were lucky to finish top after turning in a worryingly flat performance in defeat in Switzerland on matchday six, which cast further doubts on their ability to win a first European Cup since 1996 – even with 'Mr. Champions League' Ronaldo now within their ranks.
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#8 Bayern Munich
Bayern Munich's form has picked up of late, thus keeping Niko Kovac in a job, but the nature of their qualification did little to suggest that the Bavarians can claim a first Champions League since 2013.
Indeed, while the Germany champions won their group by two points from Ajax without losing a single game, they were made to look old and slow by their two draws with the young Amsterdam outfit.
Robert Lewandowski may still be at the peak of his powers – the Polish striker is the tournament's top scorer, with eight goals – but there is no denying that Franck Ribery, Arjen Robben and arguably even Thomas Muller and Manuel Neuer are well past their best.
Of course, this is still a squad packed with star names but a couple of quality signings during the January transfer window will be essential if Bayern are to be talked about as possible champions.
Ajax may have finished behind Bayern Munich but they were the more impressive of the two sides in qualifying for the last 16.
Erik ten Hag's dynamic young team demonstrated the kind of energy and attacking verve that has evoked memories of the sensational crop of players that took Europe by storm in the mid-90s.
Defender Matthijs de Ligt and midfielder Frenkie de Jong are obviously the pick of the current bunch and it is heartening that Ajax insist that neither man will be sold during the January transfer window, despite interest from all of Europe's elite in the Dutch duo.
Elsewhere, Dusan Tadic, who has five goals to his name, is in the best form of his career, but Donny van de Beek, Noussair Mazraoui and Nicolas Tagliafico have also caught the eye for this fearless and undefeated team.
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Last season's runners-up came desperately close to suffering a demoralising first-round exit, with Alisson producing a vital injury-time save in the decisive 1-0 victory over Napoli at Anfield.
One wouldn't have expected the Reds to be left needing to win their final game to progress after opening their campaign with a thrilling 3-2 victory over Paris Saint-Germain but Jurgen Klopp's were rotten on the road, losing all three matches, with the performance away to Red Star Belgrade particularly pathetic.
Liverpool's troubles on the road will probably catch up with them eventually but they remain a formidable foe on Merseyside, particularly now that Mohamed Salah is back at his brilliant best after a sluggish start to the season.
One also wonders what effect the Reds' strong bid to win a first English title since 1990 will have on their focus and fitness as the months roll on but Liverpool are undoubtedly a second seed every group winner will want to avoid.
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#5 Atletico Madrid
There were some fears that the wheels were coming off the Atletico Madrid machine after they were routed by Borussia Dortmund's young guns on matchday three.
But then the Rojiblancos went and won the return fixture at the Vicente Calderon 2-0, making reports of their demise appear grossly premature.
Still, there are some causes for concern, mainly up front, where they remain utterly reliant on France forward Antoine Griezmann for goals, as so painfully illustrated in the 0-0 draw at Bruges which cost Atleti top spot.
Nonetheless, Atleti remain a nightmare to play against – that Dortmund blackout aside – they are still pushing Barca hard in La Liga and every group winner will want to avoid them in the draw for the last 16.
#4 Borussia Dortmund
One of the stories of the European season so far, Borussia Dortmund are riding high at the top of the Bundesliga and one of the seeded teams in the draw for the Champions League last 16 after topping a group containing Atletico Madrid.
Indeed, BVB's 4-0 demolition of the usually solid Spanish side – it was the heaviest defeat of Diego Simeone's tenure to date – was arguably the most exhilarating display of the group stage.
Dortmund are formidable going forward, with Jadon Sancho proving himself one of the most exciting young players, Marco Reus resurgent, Axel Witsel in fine form, and Achraf Hakimi and Raphael Geurreiro excelling out wide. That Christian Pulisic can't get into the starting 11 says it all!
However, BVB have also been excellent defensive, conceding just two goals in six games – the best record in the competition – and there is cautious optimism around Signal Iduna park that this team can emulate the heroes of 1997.
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#3 Paris Saint-Germain
After back-to-back draws with Napoli, Paris Saint-Germain's expensively assembled side were staring a humiliating, potentially project-ending defeat in the face they hosted Liverpool on match day five.
However, PSG proved both their quality and their long-questioned mettle by beating the Reds 2-1, with Neymar outstanding going forward, and Thiago Silva and Marquinhos excellent at the back.
Liverpool's subsequent defeat of Napoli allowed PSG the nick the unlikeliest of top spots by defeating Red Star Belgrade away, meaning PSG have come out of the toughest group in the competition with their confidence restored.
Consequently, Kylian Mbappe & Co. will represent a fearsome prospect for any unseeded side in the last 16.
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#2 Manchester City
Having romped to a record-breaking Premier League title triumph last season, the Champions League is undoubtedly the primary target for Manchester City this term.
The good news is that they will be one of the seeded teams in the draw for the last 16, after topping their group with 13 points, but the bad news is that Lyon have proven that Pep Guardiola's can not only be got at, they can also be beaten if one is bold enough to go at them.
In the end, City were lucky to get even a point off the Ligue 1 outfit, having deservedly lost 2-1 at home before being battered in a 2-2 draw in France.
Still, Guardiola has outrageous strength in depth at his disposal – an injury to Kevin de Bruyne barely registered – and will be supremely confident of reaching the quarter-finals, with Ajax, Atletico, Roma and Schalke their only possible opponents in the last 16.
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This may not be the Barcelona of 2009, 2011 or 2015 but the Catalans can still call upon the services of Lionel Messi and it is his sensational form which makes them favourites for one of the most open Champions Leagues in recent memory.
The Argentine attacker stated before the season began that his goal as the club's new captain was European glory and he has looked a man on a mission thus far, scoring six goals from just three starts, including a hat-trick against PSV and a double in a wondrous display at Wembley against Tottenham.
While Barca still rely heavily on their No.10, Ernesto Valverde has taken encouragement out of the fact that they managed to take four points off Inter when Messi was out injured.
In addition, Arthur's arrival has also added more craft to the midfield and while doubts persist over both Sergio Busquets and the defence, much could hinge on whether Barca can get the best out of the talented but temperamental Ousmane Dembele, who has the pace and finishing prowess to take some of the burden off Messi.