The England international travelled down to London with the rest of his teammates on Friday and was expected to be in the starting XI against Marco Silva's side.
However, he was a notable absentee from the squad list that was submitted an hour before kickoff.
There had initially been claims that the attacker was left out for disciplinary reasons, with Red Devils head coach Erik ten Hag hitting out at last season's leading goalscorer for attending a nightclub to celebrate his birthday after the defeat to Manchester City last weekend.
However, Ten Hag confirmed to TNT Sports ahead of the game with Fulham that the 26-year-old would have started on Saturday had he been available for selection.
A statement on Man United's official website revealed that «a heavy knock to his leg in training» meant that the forward was ruled out of the contest in London.
«Rashford travelled with the rest of the team but the Mancunian forward will not be involved in west London, due to injury,» read a statement from the 20-time English champions.
«Marcus took a heavy knock to his leg in training and was unable to finish the last session before the game.
The England international underwent a fitness test this morning, when it was decided he was unable to play.»
Rashford has struggled to make his mark for Man United this season, only finding the back of the net on one occasion, which came at Arsenal at the start of September.
In his absence on Saturday, Alejandro Garnacho has been given the nod down the left, with Rasmus Hojlund and Antony also operating in the final third of the field.
Man United will be
Read on sportsmole.co.uk