The 22-year-old has barely featured for United this season, making just five appearances in all competitions.
Pereira replaced Luke Shaw on the bench on Saturday after the left-back suffered an injury in the warm-up, but the Brazil international was not brought off the bench as United sealed a 4-1 victory at Old Trafford.
United now face Valencia in their final Champions League group game on Wednesday but have already qualified for the knockout phase.
And Mourinho has revealed Pereira will be in action against the Spanish side having impressed in training.
We need these last two matches, Arsenal and today, today is really, really strong, Mourinho told MUTV.
We go to Valencia, there are people who need to rest and there are people who need to play
Its a question of trust and a question of justice, too.
If I say Romero plays, obviously he deserves to play.
If I say a kid who is working every day like a tiger and is not playing much like Andreas Pereira is going to have a start.
Im going to make changes in the team because we managed to qualify.
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