The 19-year-old, with the visitors struggling to break West Ham down, came on after 58 minutes and immediately changed the game with a superb assist for Juan Mata in the 63rd minute.
Zlatan Ibrahimovic added Man United’s second after 76 minutes, allowing Jose Mourinho’s men to enter cruise control during the latter stages.
Mata, speaking after the game, was quick to shower praise on the youngster.
“He was very good and is very good,” he said.
“He is a great talent. He can help us a lot with his pace, his goals, his assists like the one for me.
“He’s a great kid so I am very happy he is playing good football.
“It was great play by Marcus on the wing, one-against-one against the defender, and he cut back and I just tried to put it in the back of the net.”
Spare a thought for poor Håvard Nordtveit, who spent an hour dealing with both Henrikh Mkhitaryan and Jesse Lingard, only to then see Marcus Rashford come on. The right-back had also played the full 90 minutes against Leicester City just 48 hours before. Looking at it this way, there was only going to be one outcome: complete demolition.
It felt as if the Englishman’s standard aura of fearlessness and raw brilliance was this time accompanied by a veneer of cold determination. Rashford hasn’t made a start since late November, and with his team in desperate need of a goal this was the ideal time to impose himself. The result was, as anybody who watched that game will tell you, simply startling.