The Argentinian was substituted early in the second half, trotted down the tunnel and, on this evidence, should not be seen again in a City shirt.
It was not just Demichelis who was ripped apart. Here endeth, also, City's slender Premier League title hopes. Instead that hope has been replaced by a fear. A fear that City are now in a desperate fight to stave off the challenge of United and West Ham United to claim that fourth Champions League place.
This is not what Pep Guardiola signed up for next season with the City players really having to examine their woeful underperformance, with Manuel Pellegrini struggling to motivate them, and a league campaign collapsing around them when they should be taking this title. They have lost to United, Leicester, Spurs and Liverpool in the league since Guardiola's appointment was announced and drawn at Norwich City. Three of those losses were at home.
There was injury added to that insult as Raheem Sterling limped off with a groin injury and goalkeeper Joe Hart departed on a stretcher with a damaged calf.
After this weekend's results then, surely also, Leicester City, Tottenham Hotspur and Arsenal look secure in the top three ... but who can say?
This has been such an incredible, unpredictable, logic-defying season. Anyone who saw United wilt against Liverpool in the Europa League would not have expected them to pull this performance out of the bag.
Manager Louis van Gaal said that United had to win this game, or the top-four gap would have been too great, and had the delicious satisfaction of seeing his team do just that.
The chances remain that he will not be at the club next season but the blooding of Rashford will be a positive part of his dubious legacy; a young striker who has altered the mood around United.
Rashford ran riot. He scored a superbly accomplished goal, he probably should have earned a penalty, when he was barged over by Demichelis, and he would have struck in the second half had not a goal-bound shot inadvertently hit Eliaquim Mangala.
Rashford was not alone. With the excellent Anthony Martial and also Jesse Lingard, there was, for once, pace and energy to United's attack.
City can have no real grounds for complaint. They simply did not do enough. There were boos at halftime, a few at the end, but there was, more than that, a growing sense of frustration.
Telegraph Group
