From Mbappe & Messi injuries to Neymar controversy, 8 reasons why PSG enter the Champions League knockouts vs Bayern near a MASSIVE crisis

Even though Mbappe made a surprise return to training on Monday morning, his fitness should still be a massive concern for PSG. The star winger injured his hamstring on February 1, with an expected recovery time of three weeks. That he is back within 11 days is good news for manager Christophe Galtier, but also brings concern that he's been rushed.

And what kind of role will Mbappe play? It certainly seems hasty to let him start, but Galtier insisted that Mbappe isn't in the squad just to fill out numbers. A second-half introduction seems likely, but will it be worth risking their star player with a second leg coming up? And with Ligue 1 far from locked up, Galtier has a choice on his hands. An aggravation of his star player's current injury would be an absolute nightmare.

There is allegedly trouble in the PSG camp. Sporting director Luis Campos watched the squad he assembled suffer an embarrassing loss against Monaco on Saturday, and reportedly tried to intervene. According to L'Equipe, Neymar, Marquinhos and Campos got into a spat following the defeat, with Campos criticising the team for a lack of effort.

Campos typically doesn't have much influence over such matters, and the fact that the sporting director is trying to meddle in day-to-day affairs with players is an alarm bell for greater dysfunction.

What to do about Neymar? At his best, the Brazilian is unplayable, and one of the best footballers on the planet. On bad days, he's a liability. This is not to say that PSG should bench Neymar, nor that he is no longer one of the world's best. And he always seems to perform in the big games.

But personal issues seem to be impacting the team's performance at the moment. He and Mbappe have dealt with well-documented tension between them, while Neymar also reportedly had a go at Vitinha and Hugo Ekitike during the loss to Monaco on the weekend.

"It happened, a little discussion, we didn't agree," the Brazilian said in a press conference about the Monaco incident. "It happens to us every day, but I love them all, it's like with my girlfriend. Football is not just love, not just friendship. There is respect but it happens to have discussions. We are not used to losing, when there are defeats, of course it disturbs us. It is part of the process to improve."

Added head coach Galtier: "When there is frustration, it has to come out. I have been leading this way for years. You have to listen to them but they also have to listen to me. There is frustration that breeds anger. We talked, I listened to them, I hope they listened to me too."

There are two sides to every story, and Neymar has tried to paint himself as a player simply raising the level of those around him with a competitive streak. But this isn't his first internal clash. He might need to pick his battles better.

At an unstable moment for the club, stars are supposed to be leaders and maintain calm. Another drama is the opposite of what PSG wanted entering the Bayern Munich showdown.

Hakim Ziyech was never going to be the saviour of PSG's season. The winger is a fine player with a sumptuous left foot, and a hero of the Moroccan national team. But he is not the answer to all of PSG's problems.

Still, he seemed primed to be an important piece. The drop off from PSG's usual front three to Carlos Soler and Hugo Ekitike is massive. Ziyech, at the very least, should have softened the fall. But a transfer from Chelsea fell through at the last minute, with the London club failing to sort the correct paperwork before the deadline.

It left PSG without a clear option off the bench, someone who could have perhaps given Messi or Neymar a rest in the weeks leading up to the Bayern tie - or even start the contest until Mbappe is ready to enter the game.

There was a weird situation after PSG's 3-1 loss to Monaco at the weekend. Captain Marquinhos reportedly asked the team not to address the away fans. Presenel Kimpembe ignored his wishes, walking up to the traveling support with a megaphone, thanking the fans for their backing and asking them for togetherness ahead of the Bayern tie.

A typically grumpy set of Ultras applauded Kimpembe.

It seemed like a classy moment. But this is PSG, and instead, it has become a talking point, a strange subject of division in the dressing room. Marquinhos is yet to reveal why he advised Kimpembe to turn his shoulder to the away end, and if anything, the French centre-back did the right thing. But it's a curious storyline nonetheless.

Lionel Messi has been hit with the odd muscle injury as he's aged, the latest a hamstring problem revealed last week.

PSG have been careful with their superstar, keeping him out for even the slightest knock or tweak. It's hard to fault them for his fitness setbacks. Messi is well into his 30s, and probably doesn't need to play Nantes on a dreary Wednesday in January.

Still, Messi has looked particularly fatigued since the World Cup. There have been some wonderful moments, highlighted by a lovely strike to put away Toulouse in Ligue 1, but also some post-tournament exhaustion.

The club ominously ruled Messi out with a hamstring injury last week, just ahead of Tuesday's fixture. The World Cup winner was back in training Monday, and a return could soon follow. But hamstrings are notoriously susceptible to re-injury, and his fitness will have PSG holding their breath moving forward.

Injuries happen. Dressing room spats occur. But the stomach virus that hit PSG before the Monaco game seems to be an especially cruel - and rare - twist of fate.

Galtier refused to blame the result on the sickness alone, although he did admit that the ailment made things "difficult" for his side in a 3-1 defeat.

The sickness appears to have subsided by now, but the illness seems to have rattled the camp and served as another distraction for players trying to find a groove.

PSG didn't lose in the first half of the season. They weren't always entirely convincing; a couple of poor results against Benfica showed weaknesses in the side. But the Galtier revolution appeared to be well underway. His 3-4-1-2 formation seemed to be working, while Neymar was in majestic form.

Things have taken a turn for the worse in the last two months, with the club losing three times, and conceding a late equaliser in a fourth.

PSG come into Tuesday's contest on the back of two defeats, dropping points to their two biggest rivals in a concerning build-up to the biggest game of the season. Bayern haven't exactly set the world alight, but PSG are far short of their best at the moment.

External distractions aside, it's perhaps the disjointed play on the pitch that's the biggest worry for PSG supporters.