Mata strikes the hammer blow
West Ham 0 Manchester United 2
Juan Mata and Zlatan Ibrahimovic fired Manchester United to a sixth straight Premier League victory at 10-man West Ham.
United were labouring against a Hammers side holding firm despite being a man down from the 15th minute after the controversial dismissal of Sofiane Feghouli.
But Jose Mourinho sent on Mata and Marcus Rashford - who went on to claim the man-of-the-match award - at the start of the second half and the substitutes combined to break the deadlock.
Ibrahimovic, who hit 50 goals in 2016, then opened his account for the new year as United ran out 2-0 winners.
However, the defeat was harsh on West Ham, who were up against it from the moment referee Mike Dean brandished his fifth red card in 15 matches this season.
The game was spoiled as a spectacle when Algeria winger Feghouli's heavy first touch forced him into a 50-50 challenge with Phil Jones on the edge of United's area.
The tackle did not look malicious - although Jones certainly came off worse - but Dean stunned the London Stadium by producing a straight red.
United's run has seen them make significant inroads on the top four. Mourinho's men are now just a point behind fourth-placed Arsenal ahead of their trip to Bournemouth tonight and three points behind third-placed Manchester City.
West Ham manager Slaven Bilic, meanwhile, insisted two "totally unfair" decisions cost his team against United.
The Croatian said Feghouli's sending off and Ibrahimovic's "offside" goal swung the game in United's favour.
Bilic said: "It was never a red card. The other way round was the more dangerous one.
"It was Jones - he got the ball but he went with the 'scissors'. Feghouli very rarely makes a foul in the game and his foot was not high in the air.
"Phil made a meal of it, maybe because he went in a dangerous manner and to save himself.
"For me that was the crucial decision in the game, it was early and it put the game in a totally different perspective.
"That decision and the other one (Ibrahimovic's goal) were totally unfair to us and totally wrong.
"It's a big offside because the players are not sprinting. A lot of times we need slow motion, but the players were walking."
Opposite number Mourinho was unwilling to offer an opinion on Feghouli's red card and simply said: "I don't say because I was far away and I didn't watch on TV."
After substitute Mata opened the scoring, Mourinho quickly sent defender Chris Smalling into the fray, having previously switched Michael Carrick to the back to try to exploit West Ham's numerical disadvantage.
"We didn't use our superiority (in the first half)," he said. "We were playing three (defenders) sometimes against almost zero, so they had nine (outfield players), but we had seven. To get the extra player, we played Carrick to take off a centre-back and use Mata in that (midfield) position.
"We had people on the bench to bring in, Mata and (Marcus) Rashford, to bring a fresh intensity to the game.
"I knew Slaven would play with (Andy) Carroll for the last stages and Carroll is not for Carrick, he has to be for a real central defender."
Bilic rued Michail Antonio's miss when one-on-one with David De Gea shortly before Mata's opener.
He said: "They had one or two chances and when it was still 0-0, we had the best chance of the game. You know you have to score that."
Mourinho also acknowledged the importance of that moment and said: "It was a question of keeping a clean sheet, which we did because David made a super-important save with Antonio. De Gea was crucial for us."
Mourinho has also handed man-of-the-match Marcus Rashford a vote of confidence by insisting he won't be looking for another striker in the January transfer window.
"The biggest trust I give him is when I told you that I don't want another striker. I don't want one. Why? Because I have Rashford," says Mourinho.
"He's always there. He's always selected, he's on the team, he's on the bench when he doesn't start. He comes on and he has different kinds of experiences.
"What happened to him last season was really fast and was a consequence of a situation that we are not having this season.
"He's fine. He's going in the right direction. He's a good kid, a good professional with the right mentality. He's always ready to learn and improve. This kid is going in the right direction."
But having been thrust into the limelight last term is the Mancunian finding this season tough to cope with?
"No. He's intelligent. He knows he's learning every day," added Mourinho.