Juan Mata and Zlatan Ibrahimovic fired Manchester United to a sixth straight 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 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.
Yet if West Ham felt a sense of injustice they certainly could not complain about the slice of fortune which kept the sides level 10 minutes before half-time.
United looked certain to score when Ibrahimovic crossed to Henrikh Mkhitaryan, who laid the ball back across goal to leave Antonio Valencia with a simple finish at the far post.
But Valencia managed to fire the ball back towards Darren Randolph, which was quite an achievement given the Hammers goalkeeper was stranded at the opposite post.
Still, that howler appeared not to matter as Randolph scooped the ball off the goal-line, straight to the feet of Jesse Lingard two yards out.
But the winger contrived to sidefoot his effort against the post before the ball bounced back along the line and into the grateful arms of the still grounded Randolph.
Meanwhile the 10 men, having somehow repelled United for the best part of half an hour, could have gone in ahead at the interval but for a fingertip save from David de Gea to keep out Manuel Lanzini's curler.
They went even closer after half-time when Lanzini's through-ball sent Michail Antonio clean through but the winger's finish was too close to De Gea, who made the save.
That miss proved even more crucial when United made the breakthrough in the 63rd minute. Rashford skipped past the challenges of Havard Nordtveit and Pedro Obiang down the left before cutting the ball back for Mata to guide in from 10 yards out.
Ibrahimovic put the game to bed with 12 minutes left when Pedro Obiang made a hash of clearing Ander Herrera's shot and, as the West Ham defence stood and watched, the Swedish star, who looked to be in an offside position, rifled the ball home to ensure United's winning run continued.