Whisper it – and try at least for a while to ignore the enormous Brazilian ego that has temporarily vacated the room – but might Paris Saint-Germain finally have worked it out? The difference between their performances at Anfield in September and at Old Trafford on Tuesday could hardly have been more pronounced. Perhaps this PSG are contenders after all.

Certain issues, it’s true, remain. Gianluigi Buffon is 41. However agile and dominant he remains, he has the feet of a goalkeeper who grew up in the 90s; a sweeper-keeper he is not and, in a world in which almost everybody presses, that restricts how high the defensive line can play while in turn making PSG susceptible to opponents who press them.

Neither Dani Alves nor Juan Bernat are particularly defensively accomplished full-backs and, while their attacking qualities mean PSG can switch to a back three with impressive ease, Manchester United’s pace in wide areas kept threatening to undo them until Anthony Martial and Jesse Lingard were forced off halfway through the game, a double blow that, as Ole Gunnar Solskjær acknowledged, cost United a lot of their forward momentum.

Presnel Kimpembe, although he made one vital headed interception and scored the opening goal, looked a red card waiting to happen and, having been booked early, was twice fortunate to get away with offences that could easily have brought a second yellow card.

But those were relatively minor questions on a night on which PSG produced a refreshingly old-school European display. Marquinhos excelled in closing down Paul Pogba, aided by the yapping enthusiasm of Marco Verratti. Kylian Mbappé’s pace was a constant menace on the break, his finish for the second goal magnificently deft. Ángel Di María remains perhaps the best in the world at carrying the ball on the break and selecting the right option.

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After the shambles at Anfield, when Liverpool won 3-2 but could easily have scored at least double that, this was revelatory. Where that had been a complacent team, fatted on their farcical dominance of Ligue 1 and seemingly bewildered by an opponent playing with energy, aggression and ability, this was a disciplined, organised side, humble enough to dig in and accept that the main task was to stop United playing and if anything fell their way on the counter that was a bonus.

At Anfield there had been a front three, a vast gap, and then a line of overrun midfielders. When Trent Alexander-Arnold or Andy Robertson advanced, they did so unopposed. At Old Trafford PSG’s shape without the ball became a 4-4-1-1, so Ashley Young found himself facing Di María and Bernat, while Luke Shaw had both Alves and Thilo Kehrer blocking his path. From PSG, it was a classic application of two banks of four, using Julian Draxler as a central creator behind Mbappé, but with the added sophistication that it could morph into a 3-4-2-1 in possession, with Di María drifting off his flank and the two full-backs driving forward.

Neymar in action for PSG in November’s win over Liverpool, when Marco Verratti was deputed to tidy up for him. Photograph: Bertrand Guay/AFP/Getty Images

The work Thomas Tuchel has done in providing that structure is remarkable. Which leads to the biggest question of all: could PSG have played like that had Neymar been available? That seems unlikely but what is almost certainly true is that they would not have played like that. Neymar, Mbappé and Edinson Cavani started every group game apart from the away match against Napoli when Cavani came off the bench. It may be that against top-class opposition, playing that trio is not sustainable.

The sense in the group stage was of Tuchel searching for a formation: he tried 4-3-3, 4-2-3-1, 3-4-2-1 and 4-4-2. But whatever the shape, Neymar causes a problem because he does not track back consistently, which makes it almost impossible for him to be played wide. It was the same problem that cost Brazil in the World Cup against Belgium, for whom Thomas Meunier, Neymar’s PSG teammate, rampaged unchecked down that flank.

PSG were fortunate to get away with draws in both group games against Napoli. There were, it’s true, signs of solidity in the victory over Liverpool at the Parc des Princes when Neymar was deployed on the left of a 4-4-2 and Verratti deputed to tidy up for him, but the question then was what might have happened had Jürgen Klopp played Alexander-Arnold at right-back and had him attack Neymar rather than taking the more conservative option of fielding Joe Gomez there.

It’s an awkward truth to face, and whatever your opinions on how he conducts himself, there should be nothing but sympathy for Neymar as he looks to recover from his fractured metatarsal, but PSG’s absences left Tuchel with a far more balanced squad to select from. When he is fit again, the familiar existential question will rise again. What is PSG? Is it a sports-washing vehicle partnered with Nike to showcase and indulge the world’s most expensive footballer or is it, as it showed it could be on Tuesday, a seriously impressive football team?