The 20-year-old's career has been at the centre of a debate amongst pundits and critics. Some say the Englishman should leave Old Trafford in order to gain more playing time.
"He is selected for the game," Mourinho said in his pre-match news conference.
"But I just want to remind you, in advance, that we can only start with 11 players, so when tomorrow you see the team, you could try to speak about the ones who are going to play and not about the ones who aren't.
"We have [Romelu] Lukaku, Rashford, [Juan] Mata, Alexis [Sanchez], [Anthony] Martial and they cannot all play together.
"Try to be a little happy for the ones who are playing, not critical of those not playing."