The 51-year-old was not convinced by how the Red Devils turned up against Fulham.
Roy Keane has claimed Manchester United were unbelievably poor against Fulham despite a 3-1 FA Cup win on Sunday at Old Trafford.
A self-explosion by Fulham, with red cards for Willian, Aleksandar Mitrovic, and Marco Silva, gave the Red Devils a chance to score through Bruno Fernandes and Marcel Sabitzer and win to progress to the semi-final.
However, Keane, who made 473 appearances for the club, was not convinced by the performance.
"Obviously, with the FA Cup, the priority is getting into the next round, but with United, I still think their manager will be disappointed with what he saw," Keane told ITV, as quoted by Daily Mail.
"Of course Fulham self-destructed, but United were so poor it was unbelievable. I've kind of lost that little bit of confidence watching them now.
"I think about a month or two ago you're thinking 'yeah, they're up for it', but the last few games I've seen they have a real habit of, I think it's OK to play moments every now and then, but I think that seems to be their DNA at the moment."
Should the Carabao Cup winners perform the same against Brighton at Wembley in the semis, Keane is skeptical that his former side will be beaten.
"And I think they've got into some really bad habits. It's almost [like] they're turning up today [Sunday] thinking, 'we're Man United, we're going to win football matches'.
"But if they turn up with that attitude for the semi-final, Brighton will beat them. Listen, it gives us a kick up the backside."
Keane’s criticism comes after Jamie Carragher claimed Manchester United are in a real fight for the top four.
After the international break, United will face Newcastle United on Super Sunday at St James Park. On the other hand, Fulham will be back in action with a game against Bournemouth on April 1.