Match ends, West Ham United 0, Manchester United 2.
West Ham United 0-2 Manchester United
Manchester United moved level on points with fifth-placed Tottenham after victory at West Ham, who played for 75 minutes with 10 men following the controversial dismissal of Sofiane Feghouli.
Referee Mike Dean showed Feghouli a straight red card after the midfielder's 15th-minute challenge on Phil Jones.
Replays showed it was more of a coming together between two players committed to winning the ball than a reckless tackle meant to cause harm.
Antonio Valencia was guilty of an astonishing miss for the visitors before Juan Mata scored from 10 yards after a clever pass by fellow substitute Marcus Rashford.
Zlatan Ibrahimovic was one of three players offside when he doubled the lead after Pedro Obiang's clearance fell to Ander Herrera.
It was Jose Mourinho's side's sixth straight Premier League win and their seventh in all competitions.
- Relive the action from London Stadium as it happened
- Reaction from Monday's Premier League games
- Listen: 'Man Utd are back in the title race'
Dean at the centre of controversy - again
The Hammers have beaten Bournemouth, Sunderland, Burnley and Hull at home this season, yet their hopes of claiming a first major scalp at London Stadium were undone by the fastest sending off in the Premier League this season.
There is no doubt Feghouli lost control of the ball and deserved a booking for his challenge on Jones.
But Dean, who sent off Southampton's Nathan Redmond in the 4-1 defeat by Tottenham on Wednesday, brandished a red card for the fifth time this season, much to the fury of West Ham boss Slaven Bilic.
Jones, who was clearly hurt and rolled over several times before receiving treatment, was booed by home fans for the rest of the game each time he touched the ball.
Feghouli is now set to miss his side's FA Cup third-round home tie against Manchester City on Friday, while Hammers supporters showed their anger at the official by chanting 'Mike Dean - it's all about you'.
In the second half, Dean kept his cards in his pocket after Cheikhou Kouyate's reckless challenge on Henrikh Mkhitaryan.
Mourinho's substitutions do the trick
This was far from vintage Manchester United, yet Mourinho's team started 2017 as they finished 2016 - with three points.
They are now unbeaten in their past 13 games in all competitions, while they have taken 25 points from the last 33 on offer.
Valencia will surely be haunted by his 36th-minute miss. It was a brilliant save by Darren Randolph to deny him from close range, but the Ecuador international should have buried the chance, as should Jesse Lingard, who hit the post with the follow-up.
Mourinho's decisions to bring on Mata at the start of the second half and Rashford before the hour mark proved decisive.
The pair combined to break West Ham's spirited resistance - the busy and menacing Rashford evading a couple of challenges before cutting back for Spaniard Mata to find the net.
The 19-year-old England striker hit the post before Ibrahimovic, standing in an offside position, scored a controversial second to complete West Ham's misery.
Positives in defeat for Hammers
Beaten by Leicester City on Saturday, it has been a 48 hours to forget for West Ham in terms of results.
However, they dug deep, displayed a steely resolve - and might even have got something from the game despite the visitors' extra-man advantage.
David de Gea twice saved well from Manuel Lanzini, before Michail Antonio's glancing header flashed agonisingly wide as the Hammers threatened.
And shortly before Mata broke the deadlock, Antonio found himself clean through after Lanzini's perfectly weighted pass, only for De Gea to block his effort.
Man of the match - Marcus Rashford (Manchester United)
'We are champions of bad decisions'
West Ham boss Bilic: "I was pleased with the performance, we fought hard and gave everything. I told my players that if we did this we will be all right in the table.
"Ten men against a team like this is very hard - but we had chances.
"I am disappointed with the result and frustrated by how we lost it, but I am proud of my players."
Manchester United manager Mourinho: "It was hard for us to think well with one more man - and it was very hard for them physically.
"I was happy with my choices in Juan Mata and Marcus Rashford, they gave us what we needed. Rashford is very professional and very mature. He is a Manchester United player with Manchester United DNA.
"I don't feel sorry for West Ham - I didn't watch the decisions. I think if you talk about decisions, we are the champions of bad decisions."
Stats - Mourinho's longest winning run since 2014
- Manchester United are now 13 games unbeaten in all competitions - longest run since March 2013 (18 games).
- Ibrahimovic has already scored more goals in all competitions than Manchester United's top scorer last season (Martial, 17).
- This is Mourinho's longest winning run in all competitions (seven) since January 2014 when in charge of Chelsea.
- West Ham have lost consecutive Premier League games without scoring for the first time under Bilic.
- Since the start of last season, no team has been shown more Premier League red cards than the Hammers (eight - level with Southampton).
- Mata has been involved in 40 Premier League goals (25 goals, 15 assists) since his Manchester United debut. Only Wayne Rooney with 46 - 29 goals and 17 assists - has a better record in that time.
- Dean has shown 14 red cards in the Premier League since the start of last season - at least six more than any other referee.
West Ham are back in action on Friday when they host fellow Premier League side Manchester City in the FA Cup third round (19:55 GMT kick-off).
Manchester United start their defence of the famous trophy at home against Championship club Reading - managed by former Old Trafford defender Jaap Stam - on Saturday (12:30).
- 4NordtveitBooked at 90mins
- 8KouyatéSubstituted forFernandesat 82'minutes
- 10LanziniSubstituted forA Ayewat 89'minutes
- 7FeghouliBooked at 15mins
- 27PayetBooked at 29minsSubstituted forCarrollat 69'minutes
- 20A Ayew
- 1de Gea
- 25A ValenciaBooked at 75mins
- 36DarmianBooked at 42minsSubstituted forMataat 45'minutes
- 14LingardSubstituted forRashfordat 58'minutes
- 22MkhitaryanSubstituted forSmallingat 65'minutes
- Mike Dean
- Shots on Target
Second Half ends, West Ham United 0, Manchester United 2.
Håvard Nordtveit (West Ham United) is shown the yellow card for a bad foul.
Marcus Rashford (Manchester United) wins a free kick in the defensive half.
Foul by Håvard Nordtveit (West Ham United).
Attempt blocked. Zlatan Ibrahimovic (Manchester United) right footed shot from outside the box is blocked.
Substitution, West Ham United. André Ayew replaces Manuel Lanzini.
Hand ball by Winston Reid (West Ham United).
Corner, Manchester United. Conceded by Darren Randolph.
Attempt saved. Ander Herrera (Manchester United) right footed shot from the centre of the box is saved in the bottom right corner. Assisted by Marcus Rashford.
Substitution, West Ham United. Edimilson Fernandes replaces Cheikhou Kouyaté.
Paul Pogba (Manchester United) wins a free kick in the defensive half.
Foul by Pedro Obiang (West Ham United).
Attempt blocked. Andy Carroll (West Ham United) header from the centre of the box is blocked. Assisted by Michail Antonio with a cross.
Attempt saved. Zlatan Ibrahimovic (Manchester United) right footed shot from the left side of the box is saved in the bottom right corner. Assisted by Marcos Rojo.
Foul by Paul Pogba (Manchester United).
Pedro Obiang (West Ham United) wins a free kick in the attacking half.
Goal! West Ham United 0, Manchester United 2. Zlatan Ibrahimovic (Manchester United) right footed shot from the centre of the box to the bottom left corner. Assisted by Ander Herrera.
Attempt blocked. Ander Herrera (Manchester United) right footed shot from outside the box is blocked. Assisted by Paul Pogba.
Attempt saved. Marcus Rashford (Manchester United) right footed shot from the right side of the box is saved in the bottom right corner. Assisted by Zlatan Ibrahimovic.
Antonio Valencia (Manchester United) is shown the yellow card for a bad foul.
Foul by Antonio Valencia (Manchester United).
Manuel Lanzini (West Ham United) wins a free kick on the left wing.
Attempt missed. Paul Pogba (Manchester United) right footed shot from outside the box is close, but misses to the right. Assisted by Marcus Rashford.
Foul by Phil Jones (Manchester United).
Andy Carroll (West Ham United) wins a free kick in the attacking half.
Substitution, West Ham United. Andy Carroll replaces Dimitri Payet because of an injury.
Attempt missed. Paul Pogba (Manchester United) left footed shot from outside the box is close, but misses to the left. Assisted by Ander Herrera.
Substitution, Manchester United. Chris Smalling replaces Henrikh Mkhitaryan.
Paul Pogba (Manchester United) wins a free kick in the attacking half.
Foul by Cheikhou Kouyaté (West Ham United).
Goal! West Ham United 0, Manchester United 1. Juan Mata (Manchester United) left footed shot from the centre of the box to the centre of the goal. Assisted by Marcus Rashford.
Offside, Manchester United. Juan Mata tries a through ball, but Zlatan Ibrahimovic is caught offside.
Attempt saved. Michail Antonio (West Ham United) right footed shot from the centre of the box is saved in the centre of the goal. Assisted by Manuel Lanzini with a through ball.
Paul Pogba (Manchester United) wins a free kick on the left wing.
Foul by Michail Antonio (West Ham United).
Substitution, Manchester United. Marcus Rashford replaces Jesse Lingard.
Foul by Phil Jones (Manchester United).
Manuel Lanzini (West Ham United) wins a free kick in the attacking half.
Attempt missed. Michail Antonio (West Ham United) header from very close range misses to the left. Assisted by Dimitri Payet with a cross following a set piece situation.