Match ends, Manchester United 1, Sheffield United 0.
Wayne Rooney scored a last-minute penalty as a poor Manchester United edged League One Sheffield United to reach the FA Cup fourth round.
The hosts, 47 places above their opponents in the football pyramid, had only two shots on target throughout.
Substitute Memphis Depay curled just wide in the second half, but it looked like the visitors would earn a replay.
However, after Memphis was fouled inside the area, Rooney stepped up to score and send the hosts through.
Van Gaal's 'Mark Robins moment'?
Just two weeks ago Louis van Gaal's future as Manchester United manager appeared to be in huge doubt.
The Red Devils had failed to win in seven games but, after surviving a 2-0 defeat at Stoke on Boxing Day, Van Gaal has overseen an improvement in both results and, in patches, performances.
After a combined 31 shots on goal in their last two games against Chelsea and Swansea, Manchester United seemed a livelier prospect, despite their plentiful possession still having no end product.
And, at Old Trafford on Saturday, Memphis fizzed an effort just wide in the second half, only for the home fans to burst into ironic cheers, rather than roar encouragement.
A drab draw against a League One side would likely have seen Van Gaal come under criticism once again, but Rooney's strike could give him some breathing space.
Back in 1990, Mark Robins scored a goal in the FA Cup for Manchester United that is said to have saved Sir Alex Ferguson from the sack.
Van Gaal may have just had his 'Robins moment'.
'I'd get rid of Van Gaal'
Sheffield United looked to have secured a lucrative replay at Bramall Lane but, as the game entered the 90th minute, Memphis broke into the area and fell as he was challenged by midfielder Dean Hammond.
His acrobatic reaction suggested the hosts may have got lucky when referee Jon Moss pointed to the spot, but replays showed there was limited contact.
"I thought it was a penalty but it was an absolute shocking performance from Manchester United," BBC Radio 5 live pundit Robbie Savage said.
"For me, Van Gaal, I'd get rid of him."
Blades caution proves costly
Two years ago Sheffield United reached the semi-finals of the FA Cup and had forged a reputation as seasoned giant killers.
In recent years, they have beaten Aston Villa, West Ham and Southampton in cup competitions and, given the sense of unrest at Old Trafford, they will have fancied their chances of claiming a higher division scalp.
They had the game's first shot on target, which came in the 67th minute after Chris Basham fired straight at David De Gea, but retreated into their own half as they looked to keep Manchester United out and take the tie back to South Yorkshire.
That perhaps proved their undoing as the hosts pressed in the closing stages, leading to the moment when Memphis was brought down inside the box.
A little more conviction, and the result could have been different.
Man of the match - Memphis Depay
'Man Utd players looked bored'
"There just seems to be an acceptance of bad performances," former Manchester United midfielder Paul Scholes told BT Sport.
"Every time you come to Old Trafford you see negative football."
What they said
Manchester United manager Louis van Gaal: "You have to see also how the opponent is playing. Everybody shall give applause to Sheffield United. They were very organised but it is much more difficult to attack than to defend.
"Again a clean sheet is also the benefit of our organisation. I'm very disappointed that we are not creating too many chances, that we have to improve."
Sheffield United boss Nigel Adkins on BT Sport: "The players have worked extremely hard against superior opposition.
"The grit, desire and character gave the supporters something to cheer about. The supporters have got right behind the players and there is a togetherness at the football club."
The stats you need to know
- Wayne Rooney has scored nine goals in his last 10 FA Cup games for Manchester United.
- Manchester United have lost once in 37 FA Cup ties against lower-ranked sides - v Leeds United in 2010.
- Manchester United have progressed from 29 of their last 31 third round ties in the FA Cup.
Manchester United travel to Newcastle in the Premier League on Tuesday, while Sheffield United are back in League One action as they face 2013 FA Cup winners Wigan.
The draw for the FA Cup fourth round takes place on Monday and is live on The One Show on BBC One from 19:00 GMT.
- 1de Gea
- 27FellainiSubstituted forPereiraat 78'minutes
- 8MataSubstituted forDepayat 60'minutes
- 21HerreraSubstituted forLingardat 60'minutes
- 19McEveleySubstituted forReedat 78'minutes
- 18SammonSubstituted forDoneat 90+1'minutes
- 10SharpSubstituted forFlynnat 64'minutes
- Jonathan Moss
- Shots on Target
Second Half ends, Manchester United 1, Sheffield United 0.
Foul by Jesse Lingard (Manchester United).
Martyn Woolford (Sheffield United) wins a free kick in the defensive half.
Memphis Depay (Manchester United) wins a free kick in the defensive half.
Foul by Chris Basham (Sheffield United).
Attempt blocked. Ryan Flynn (Sheffield United) right footed shot from outside the box is blocked. Assisted by Louis Reed.
Bastian Schweinsteiger (Manchester United) wins a free kick in the defensive half.
Foul by Neill Collins (Sheffield United).
Goal! Manchester United 1, Sheffield United 0. Wayne Rooney (Manchester United) converts the penalty with a right footed shot to the bottom left corner.
Penalty Manchester United. Memphis Depay draws a foul in the penalty area.
Penalty conceded by Dean Hammond (Sheffield United) after a foul in the penalty area.
Delay over. They are ready to continue.
Substitution, Sheffield United. Matt Done replaces Conor Sammon because of an injury.
Delay in match Conor Sammon (Sheffield United) because of an injury.
Attempt missed. Memphis Depay (Manchester United) right footed shot from outside the box is close, but misses to the left. Assisted by Daley Blind.
Attempt blocked. Anthony Martial (Manchester United) left footed shot from the centre of the box is blocked.
Anthony Martial (Manchester United) wins a free kick in the attacking half.
Foul by Chris Basham (Sheffield United).
Jesse Lingard (Manchester United) wins a free kick on the right wing.