United's chances of winning the Premier League took a blow after a home loss to the bottom-placed Blades.
And Ferdinand says his former side were not helped by an elusive forward line.
"They didn't have that zip about them. From minute one, really, there wasn't that cut and thrust we've seen from United," Ferdinand told BT Sport after the final whistle.
"Credit Sheffield United, Chris Wilder set them up very well, but United didn't ask the questions. Bruno Fernandes, who they've relied on so heavily in recent weeks, months, didn't get it going today. Paul Pogba the same.
"If anybody was starting to get carried away at Old Trafford, with the position that they're on, it was a reality check. I think recently Man Utd, in terms of reaction, have been brilliant but tonight it seemed to come crumbling down.
"Rashford and Martial were invisible, didn't get on the end of anything, didn't create anything."