An emotional Louis van Gaal dubbed his side's performance against Arsenal as one of the worst he has ever seen at the club under his management.
The Red Devils were blown away by Arsenal on Sunday, with Alexis Sanchez and Mesut Ozil scoring a goal apiece within seven minutes. Things got worse for the visitors a few minutes later when they found themselves 3-0 down after just 19 minutes of the match had been played.
Despite dominating possession after the break, United failed to create much against a solid Arsenal outfit, and manager Van Gaal revealed his disappointment after the encounter.
"When you start like that, you can never win a game," he told Sky Sports.
"We lost every duel, every second ball, were not tight on their midfielder and they can play. They are a fantastic team, when you give them that space, it is difficult.
"That was the worst United have been under me. We were not able to give a normal performance.
"I have never seen that of my team, that is the question I put to my players. You cannot solve that now – a lot of emotion and I am emotional as well.
"I could not change anything. If it was just one player, then I can take measure, but it was more than half of my team who were not doing what we had agreed in our game plan.
"I am shocked. You don't expect it. We were top of the league, won our last four matches and have to protect our position against Arsenal.
"The way we had started, so without emotion. I did not expect that. It is difficult to understand and I will have to look for the reasons."
The loss saw the Red Devils drop down from second to third position in the league, with Arsenal moving up two spots into second place.
What do you think went wrong for Van Gaal and United on Sunday? Have your say in the comments section below.