Maybe Ole Gunnar Solskjaer will try to defend his decision not to substitute Fred long before he was inevitably sent off, and Fred may well try to justify one of the most brainless of performances in a Manchester United shirt but, whatever is said, this defeat has left their place in the last 16 of the Champions League in jeopardy.
United travel to RB Leipzig next week having to avoid defeat to progress, knowing only a point here against Paris Saint-Germain would have been good enough. If they lose in Germany they are out. They had their chances, but PSG will argue Fred should have been dismissed long before he was sent off for a series of fouls. A first-half headbutt on Leandro Paredes should have been a straight red for Fred.
Solskjaer admitted his midfielder was lucky to stay on the pitch at that point and he considered taking him off at half-time, which would have saved Fred from a second yellow card with 20 minutes remaining when he clumsily collided with Ander Herrera.
"Fred shouldn't put his head towards him. (But) I don't think he touched him. He was a bit lucky to stay on," he said.
"We spoke (at half-time) about staying calm and on your feet. The second yellow card was nowhere near a foul. Ander (Herrera) knows that."
There was further concern for United, with goalscorer Marcus Rashford substituted and walking down the tunnel with an apparent injury. Against his old side, Edinson Cavani went close, striking the goal-frame, but PSG can go through if they beat Istanbul Basaksehir while United fight it out with Leipzig.
Leipzig's late win at Istanbul Basaksehir prior to kick-off piled the pressure on PSG, who reacted like a team knowing they needed victory as Neymar and Kylian Mbappe quickly combined.
They exchanged passes, Mbappe ran at the United defence, beating Scott McTominay, with his shot deflecting up off Victor Lindelof for Neymar to seize upon and fire a powerful half-volley that beat David de Gea at his near post.
Fred sloppily cleared the ball straight to Alessandro Florenzi soon after, and he forced De Gea into a save from 25 yards. The PSG right-back was involved again as Neymar only just failed to reach his cross as Aaron Wan-Bissaka failed to react. By 15 minutes in, PSG had claimed 70pc of the possession.
When United did venture into PSG territory they were almost undone by a counter-attack as Mbappe played a one-two with Neymar before trying to tee up the unmarked Moise Kean, only to overhit his pass. Neymar eventually hooked a shot goalwards, which De Gea held.
United then got lucky, with Italian referee Daniele Orsato, despite checking the Var screen, somehow only cautioning Fred, even though he clearly pushed his head into Leandro Paredes's face before the PSG midfielder fell dramatically to the turf.
It got heated, with players crowding around Fred, and followed United claiming Bruno Fernandes had been elbowed in the face by Danilo (he had not). PSG were incensed.
United capitalised with another slice of fortune, as Danilo tried to block Rashford's shot, which Keylor Navas would have covered, only to divert it into his own net.
Once again Rashford - or was it an own goal? - had scored against PSG, who will have felt aggrieved that Paredes was then booked for a challenge on Fred when the United midfielder trod on his foot. The caution should have been for Fred, which would have resulted in his dismissal.
It continued to flow from end to end as PSG flashed crosses in, one of which Mbappe only just failed to reach, and United threatened more.
"F****** aggression!" shouted captain Harry Maguire and there was an edge to the match, with Neymar claiming he had been trodden on by McTominay.
At half-time, Neymar had to be pulled away from confronting McTominay. The stakes were high.
So were the emotions. It was a surprise to see Fred not replaced at half-time given he was walking a tightrope - one he should have fallen off - but it was United who missed a golden chance to go ahead when Rashford broke down the right, superbly exchanged passes with Cavani and crossed for Anthony Martial, who side-footed over from six yards. United players held their heads in disbelief.
Rashford then provided for Cavani, but his angled shot went across goal. The Uruguayan was then desperately unfortunate as he latched on to Rashford's pass and delicately chipped Navas outside the area, with the ball rebounding off the crossbar.
Fernandes then flicked the ball to Martial, whose powerful low drive was brilliantly blocked by Marquinhos.
Somehow PSG escaped before they finally threatened as Neymar picked out Florenzi, who lifted the ball across the area for Marquinhos, whose header clipped the bar.
Mitchel Bakker then forced a smart low save by De Gea. From the corner the ball was cleared to Herrera, whose shot was going wide only for Abdou Diallo to stretch and stop it before sending it into the six-yard area where Marquinhos, just onside, controlled it and stabbed past De Gea.
Incredibly, moments later, Fred was sent off for a tackle on Herrera, earning him a second yellow card. It was actually the cleanest of his reckless challenges.
United had to score, but Mbappe should have ended any doubt as he sprinted from his own half, clear on goal, only to shoot narrowly wide.
Neymar did finish it when PSG broke again, substitute Rafinha squaring the ball for the Brazilian to sweep home.
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