Match ends, Manchester United 0, Southampton 1.
Substitute Charlie Austin scored seven minutes into his Southampton debut as a lacklustre Manchester United were booed off at Old Trafford.
Austin rose to head home a free-kick in the 87th minute to win an otherwise dour game of few chances.
Southampton have now won at United in the league for the last two seasons.
United and their manager Louis van Gaal were jeered by home fans at full-time after producing just one shot on target in 90 minutes.
They have won just one of their past five home games - and have failed to score in three of those.
United's first-half record in front of goal is even worse. They have now gone eight hours and 26 minutes without scoring in the first 45 minutes of games at Old Trafford.
Van Gaal's side are now five points behind fourth-placed Tottenham, who won 3-1 at Crystal Palace on Saturday.
Southampton are now just four points and three league places behind United, having won their past three matches. The win also ends a four-match losing streak away from St Mary's for Ronald Koeman's team.
- Follow all of Saturday's post-match reaction here
- Relive Saturday's action from Old Trafford
- Football Daily Podcast: Premier League round-up
Austin's dream debut
Southampton paid QPR just £4m to sign Austin on 16 January - a figure more representative of the length of time left on his contract with the Championship club than his ability.
In the 2014-15 season, the striker scored 18 Premier League goals, then added 10 in 16 Championship matches prior to his move back to the top flight.
And it took him just seven minutes to pick up where he left off as he found space in the box to meet fellow substitute James Ward-Prowse's free-kick and bury a header past David de Gea.
Prior to that, the Saints were comfortable, working hard to disrupt United's rhythm, managing possession well and looking to hit on the break.
They produced the best of what were few chances in the game, but Sadio Mane fluffed a shot from the edge of the box and Shane Long and Victor Wanyama headed wide from successive corners.
Tame United fail to threaten
United came into the game off the back of two encouraging performances - an improved attacking display at Newcastle and a confidence-boosting win at Liverpool.
But they were unable to build on those, producing a tame 90 minutes strewn with errors and completely lacking in speed of thought or movement.
It made Southampton's containment job an easy one and left them vulnerable to the kind of late sting they had inflicted at Anfield last Sunday.
Van Gaal and his team were booed off at the end of a first half in which Saints keeper Fraser Forster was only tested once - from an optimistic 40-yard shot from Daley Blind - and Wayne Rooney, Anthony Martial and Jesse Lingard all shot woefully wide.
The jeers and abuse were far more ferocious at the end of a second half that United began more encouragingly following the introduction of Juan Mata, but quickly faded after Southampton had seen off their initial charge.
Is this the end?
The defeat is sure to intensify the pressure on Van Gaal.
The 64-year-old had suggested last week's win at Liverpool could be "fantastic starting point" for his side in 2016, but he has now led United to their lowest ever Premier League points tally (37) after 23 games.
After the match, the Dutchman admitted that the home supporters were right to boo, adding: "In better or worse we have to stick together.
"You have to meet expectation and expectations are very high, that gives a lot of pressure for the players.
"We are working very hard but we have a lot of injuries. That you cannot change."
Man of the match - Victor Wanyama
The stats you need to know...
- Southampton have won back-to-back away games at Old Trafford against Man United for the first time since 1969.
- Charlie Austin has scored four times in his last five league games. It was his first Premier League goal since the final day of last season, meaning he's scored in back-to-back Premier League games.
- Old Trafford has seen just three first half goals scored in 11 Premier League games this season; fewer than any other stadium. Only one of those has been scored by a United player (Memphis v Sunderland).
- United have managed just one shot on target in their last two home league games to Southampton.
What they said...
Manchester United manager Louis van Gaal: "I had a feeling they could score only from a set-play. Our defence in set-plays was weaker because I changed Fellaini and had to change Darmian. Every change was a weakness in defence.
"I wanted to win this game and that's why I changed Cameron Borthwick-Jackson for Adnan Januzaj. That is a risk you take, it was a poor game and we didn't create so much and neither did our opponent."
Southampton manager Ronald Koeman: "We played very good. Our organisation was good, we were very compact, we had good pressing.
"We stayed in the game, controlled the game and you saw more nerves in the Man Utd team and finally we score from a corner kick.
"We showed spirit today, it was an amazing result for the second time here."
Manchester United are away at Championship side Derby in the fourth round of the FA Cup next Friday evening. Southampton's next match is in the league - a tough trip to Arsenal on Tuesday, 2 February.
- 1de Gea
- 36DarmianSubstituted forMcNairat 60'minutes
- 27FellainiSubstituted forMataat 45'minutes
- 43Borthwick-JacksonSubstituted forJanuzajat 86'minutes
- 17van Dijk
- 6José Fonte
- 2Cédric Soares
- 4ClasieSubstituted forRomeuat 67'minutes
- 11TadicSubstituted forWard-Prowseat 67'minutes
- 10ManéSubstituted forAustinat 79'minutes
- Mike Jones
- Shots on Target
Second Half ends, Manchester United 0, Southampton 1.
Attempt missed. Adnan Januzaj (Manchester United) left footed shot from the left side of the box is close, but misses to the right.
Hand ball by Shane Long (Southampton).
Attempt saved. Oriol Romeu (Southampton) left footed shot from the left side of the box is saved in the bottom left corner.
Foul by Wayne Rooney (Manchester United).
Virgil van Dijk (Southampton) wins a free kick in the defensive half.
Goal! Manchester United 0, Southampton 1. Charlie Austin (Southampton) header from the centre of the box to the bottom right corner. Assisted by James Ward-Prowse with a cross following a set piece situation.
Foul by Adnan Januzaj (Manchester United).
Charlie Austin (Southampton) wins a free kick on the right wing.
Substitution, Manchester United. Adnan Januzaj replaces Cameron Borthwick-Jackson.
Attempt missed. Shane Long (Southampton) header from the centre of the box is too high. Assisted by James Ward-Prowse with a cross following a set piece situation.
Substitution, Southampton. Charlie Austin replaces Sadio Mané.
Foul by Chris Smalling (Manchester United).
Shane Long (Southampton) wins a free kick on the right wing.
Offside, Manchester United. Cameron Borthwick-Jackson tries a through ball, but Juan Mata is caught offside.
Ander Herrera (Manchester United) wins a free kick in the attacking half.
Foul by Victor Wanyama (Southampton).
Corner, Southampton. Conceded by Jesse Lingard.
Foul by Jesse Lingard (Manchester United).