United were insipid as an attacking force for 93 minutes at Old Trafford, but found the breakthrough at the death as all three of Ralf Rangnick’s substitutes combined when Anthony Martial and Edinson Cavani teed up Rashford for a tap in at the back post.
Victory lifts United a point above the Hammers and sees the Red Devils leapfrog Tottenham and Arsenal in the battle for Champions League football next season.
Rangnick has lost just one of his 10 games since taking temporary charge till the end of the season, but once again the result was more impressive than the performance from the German’s side.
Bruno Fernandes looked back to his best with two goals and two assists in the past week and the Portuguese nearly created the opener with a fiercely struck cross that just evaded Cristiano Ronaldo in the most dangerous moment of the first half.
Alphonse Areola was drafted in to the West Ham goal in the absence of Lukasz Fabianski due to a positive test for coronavirus, but the Frenchman was forced into just one serious save when he turned Fred’s driven shot behind early in the second half.
Fernandes dragged another effort wide from the edge of the box as United toiled to the frustration of the 75,000 crowd.
However, Rangnick’s changes made the difference.
Rashford has looked devoid of confidence in recent months, but his first goal since October in a 3-1 win at Brentford on Wednesday gave the England international a lift.
Rangnick claimed that Anthony Martial had refused to be part of his squad for last weekend’s 2-2 draw at Aston Villa as he looks for a move away from the club for more game time.
The Frenchman publicly denied that accusation and was given his first minutes under the interim boss when he was introduced along with Cavani in the final 10 minutes.
David de Gea was nearly caught out by a long-range effort from Declan Rice as the game entered stoppage time.
But rather than snatch a winner, West Ham were hit with a sucker punch on the counter-attack.
Martial played in Cavani on the left of the box and the Uruguayan’s cross just required a touch from Rashford at the back post.
The wild celebrations survived a VAR check against Cavani for offside and there was barely time for West Ham to kick-off in what could be a fatal blow to their outside hopes of reaching the Champions League for the first time.