United were 1-0 up at half-time in the game, but lost 3-1 to the reigning treble winners.
Marcus Rashford felt he was fouled by Kyle Walker, moments before City went up the other end and equalized through Phil Foden.
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Analysing the incident on Sky Sports' Ref Watch, former Premier League referee Dermot Gallagher stated: "The referee is close, the referee said there was no foul.
"If VAR overruled the referee there, that is clearly re-refereeing and not a clear and obvious error.
I didn't think it was a foul, he went down very easily but it would have been too far back as well (to overturn the goal)."