“I must confess I don’t know him,” the City playmaker said at the pre-match press conference. “I don’t watch a lot of football when I’m at home.”
But what’s certain is that De Bruyne’s manager Pep Guardiola knew all about the latest French attacking prospect long before he scored against his team. As far back as December 2015 – when Guardiola was being revealed as the City commander-in-waiting – German newspaper Bild was carrying a report linking Pep’s Bayern Munich with the player described as the “new Franck Ribery”.
And Bayern weren’t the only ones. Inevitably, their rivals Borussia Dortmund were also credited with interest in the same report.
As far back as 2013, Real Madrid were sniffing around with Zinedine Zidane himself giving the young Mbappe a guided tour of their facilities. At that stage he wasn’t ready to leave his homeland and joined Monaco instead even though Paris Saint-Germain were keen.
Chelsea, too, were all over Mbappe before he broke out while Arsene Wenger has lately admitted his admiration for the boy he compares with Thierry Henry.
While De Bruyne - and all of those who don’t watch French football too closely – might have been taken by surprise by the impact Mbappe made against City, those making the decisions at Europe’s elite clubs know him back to front. Indeed, it has been reported by L’Equipe that Guardiola failed in a €40 million bid to bring Mbappe to the Etihad Stadium last summer.
The 18-year-old signed his first professional contract last March, tying him to Monaco until 2019, but before that Liverpool made an attempt to snatch him for a small training fee. His services have been safely secured for now, with bids for him likely to be in excess of €80m when Monaco are ready to cash in. But for the player and the club, the time is not right.
Monaco are a club with a strong reputation for developing top talent - in recent seasons they have sold Anthony Martial, Layvin Kurzawa, Geoffrey Kondogbia and Yannick Ferreira Carrasco to some of the biggest clubs in the world and it is proving to be the ideal nursery for Mbappe, too.
“I have a clear idea of what I want to do,” Mbappe told RMC earlier this season. “I’ve had a plan for my career since I was young. My family and my club have prepared me for all this. I know where I’m going and I won’t let anything disturb me.”
He is taking giants steps in the principality with a clear development plan in front of him for good measure. In short order he has become Monaco’s youngest-ever player, youngest-ever goalscorer and the second-youngest Frenchman ever to score in the Champions League after Karim Benzema.
Mbappe is not strictly competing for game time with Valere Germain, Radamel Falcao and Thomas Lemar but is being given it when required - though his father and agent, Wilfried, is not necessarily too pleased about that.
He told L’Equipe earlier this season that Kylian hadn’t stuck around at Monaco in order to be a fifth or sixth-choice striker; he could have done that at one of the European giants.
Those quotes came hot on the heels of his son’s breakout international tournament. While Europe was winding down after Euro 2016, the European Under-19 Championship was kicking off in Germany. France carried the day, with Mbappe weighing in with five goals through the competition - including two and an assist in the semi-final against Portugal.
In fairness, there were mitigating circumstances to his lack of minutes at club level. An early-season concussion suffered against Guingamp on the opening day restricted his game time to eight minutes out of the next 720, but 2017 has so far proven to be the year of Mbappe.
A hat-trick earlier this month against Metz put him in the firmament, with those Henry comparisons looking apt given the quickness and cold-blooded finishing on display. He became the youngest player to score a Ligue 1 hat-trick since Jeremy Menez 12 years ago. It was his second treble of the season, following one in the 7-0 destruction of Rennes in the Coupe de la Ligue in December.
His goal against City - a bent run and a ruthless shot past Willy Caballero - was evidence of his blossoming maturity. He has averaged a goal or assist every 55 minutes in Ligue 1.
For those reasons and more Mbappe is the hottest name in town. Only a select few clubs will be able to afford him when he departs Monte Carlo - and depart he surely will. For his own good, however, it should not be this summer.
Mbappe only has to look at the fate of Martial to see what happens when a move comes too early or with too much expectation attached. Martial was sold by Monaco for a fee that could rise to over £60 million with an immediate burden for performance thrust upon him at Manchester United. He is still not quite coping with it despite now having turned 21. His development has been stunted.
Monaco are being smart with Mbappe, conserving his minutes and ensuring he is ready to strike, generally, against the French league’s lesser lights. By next season he will be a key player and ready to lead the line full-time. A crucial season beckons.
There is a strong possibility that Radamel Falcao will depart this summer - he is Monaco’s highest earner and a hangover from their brief foray into signing readymade stars - and Mbappe will step into the front line.
Monaco, by their own admission, are a selling club but are content to do the selling only on their own terms. This year it will likely be Bernardo Silva, Fabinho and Djibril Sidibe who will leave.
The silky Silva will fetch a price of €80m regardless of his destination, with Chelsea strongly linked with his signature. Fabinho will go for between €40m and €50m, with his versatility impressing Manchester United among others.
Sidibe - who turned down Arsenal last summer - will bring in €20m to €30m and fits the profile for Barcelona. There is no onus on Monaco to cash in on Mbappe as a result, particularly as they are overwhelmingly likely to be in the Champions League again next season either as winners of the league or by finishing within its top three.
No matter what happens, though, there is no doubting that De Bruyne knows who Kylian Mbappe is now.
Additional reporting by Julien Quelen, Goal's Ligue 1 correspondent.