Ole Gunnar Solskjaer cut a frustrated figure after Chris Wilder led rock-bottom Sheffield United to a stunning Premier League win at Old Trafford.
The title-chasing Red Devils were on the end of one of the biggest shocks in a surprise-packed campaign as the embattled Blades inflicted the hosts' first top-flight defeat since November 1.
Ex-Manchester City youth prospect Kean Bryan opened the scoring in a first half in which Anthony Martial saw an equaliser ruled out for an apparent foul by Harry Maguire on Sheffield United goalkeeper Aaron Ramsdale.
Skipper Maguire did eventually find a leveller in the second half but Solskjaer's comeback kings floundered rather than rallied.
The Blades capitalised on some atrocious defending as substitute Oliver Burke struck home off of Axel Tuanzebe to seal a 2-1 win – only their second Premier League triumph of a wretched season.
"Well, of course we're disappointed," Red Devils boss Solskjaer said. "Any result in the Premier League this season shouldn't surprise anyone. But then again, very disappointed we couldn't put a better performance on.
"It was down to us to put a performance up and we just couldn't find an opening behind or round or between a very compact and good Sheffield United defence.
"The big moments went against us. If you look at it, Billy Sharp's been around the place and he knows how to put a goalkeeper off and that's a definite foul on him (before Bryan's opener), so David (De Gea) can't get up.
"For me, the referee makes a mistake by blowing the whistle (for a foul by Maguire) on our goal (from Anthony Martial in the first half) but that happens."
Solskjaer was annoyed by the poor defending in the build-up to Burke's heavily-deflected winner and knows his side need to get their mojo back quickly.
"That little spark, that edge, we didn't have that extra creativity, imagination, the X factor we've had," the Norwegian said.
"It's happened, we look at it, agree it is not good enough and go again."
Solskjaer's men return to action at Arsenal this weekend, while the Blades head to Manchester City buoyed by a hard-fought win against the odds.
"I think I'm always a proud man being involved in this football club of ours," Blades boss Wilder said.
"A frustrated man at times as well for the majority of the season because I don't think our performances have sort of yielded the results that we should have got. But tonight was a good performance and obviously a huge three points for us."
This was the Blades' first win against Manchester United since 1993, while Wilder became the first manager to lead the club to victory at Old Trafford since 1973.
"Personally, it's all about a result for a team that has had a tough season so far," the boyhood Sheffield United fan said.
"I've just been told about (1973), TC [Tony Currie] scored the winner.
"It's great for the players because it's been a real difficult season. Some stuff that we've not got right, some stuff that we could have controlled and a lot of stuff that was out of (our) control.
"You look at the team tonight, you look at Max Lowe and Lys Mousset (on the bench) possibly could only do 10 minutes so we only had three subs.
"So it's that type of attitude and identity that makes me proud and delights me. We had to show all those qualities tonight."
