Both players withdrew from international duty in order to nurse respective injuries ahead of a busy period.
Fellaini has recently recovered from injury, while Martial has scored six goals in his last five games.
Speaking at a press conference ahead of the meeting with Palace at Old Trafford, Mourinho confirmed that the pair would play.
Both Martial and Fellaini have been training at Carrington this week and, while probably not 100 per cent, will need to be called on amid uncertainties surrounding the fitness of so many players.
The Belgian’s inclusion in midfield most likely comes at the expense of Paul Pogba, who has not trained with the team this week, while the idea of anyone other than Martial playing on the left hand side seems preposterous.
Martial has discovered new depths of strength over the last month at a time when others are floundering going forward – much like when he first joined in 2015 – and supporters will, no doubt, expect him to lead the attacking onslaught against Roy Hodgson’s side.