Mike Phelan was left furious after mistakes from Victor Lindelof and David de Gea allowed Chelsea to open the scoring in their FA Cup semi-final on Sunday.
Manchester United, who had not lost since January, eventually fell to a 3-1 defeat against the Blues.
Ole Gunnar Solskjaer's side have been impressive in recent months but a catalogue of errors meant they were never in the game at Wembley.
The opening goal especially bought about an angry reaction from the United No 2.
Phelan was seen gesturing on the bench after Lindelof was slow to react, allowing Olivier Giroud to flick the ball goalwards.
But De Gea has also been criticised after his weak hand wasn't enough to stop the ball trickling in.
Phelan was clearly unimpressed as he sat next to Solskjaer and Kieran McKenna.
But things didn't improve as Mason Mount doubled Chelsea's lead early in the second-half, with his strike from distance going straight through De Gea.
The Spaniard has been error prone for more than a year now and copped yet more criticism.
Harry Maguire's own goal compounded things as Bruno Fernandes' penalty was nothing more than a consolation.
But Solskjaer was quick to come out and defend his No 1, insisting he will not be dropped.
"[David] knows he should save that one 100 times out of 100 but that's football for you," said the United boss.
"It is done. Move on. I know he is a mentally strong person and goalkeeper.
"I can't speak for his confidence but he is mentally very strong. Everyone has to perform and has a chance to stake a claim every time they play.
"It is hard for a keeper to make amends, apart from David making two or three fantastic saves."
