Pereira has spent the majority of the season warming the bench and has only taken to the pitch five times.
The Brazil international played his first full match of the season earlier today in United’s 4-1 victory over Fulham.
Pereira was drafted in as a last-minute replacement for England left-back Luke Shaw, who suffered an injury in the warm-up.
And Mourinho has revealed Pereira will deputise for the England international in United’s final Champions League group game.
“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.
“It’s 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.
“I’m going to make changes in the team because we managed to qualify.”
Mourinho’s troops sealed passage through to the last 16 of Europe’s elite club competition last month with a 1-0 victory over Young Boys.
United go into Wednesday’s match on the back of their first league win in over a month, which Mourinho was delighted with.
“The first half was really good. We gave no chances to Fulham,” Mourinho said. “We were strong, aggressive and intense. At times it was beautiful football. The first half was perfect.
“The second half was Ranieri’s fault. He made two changes that improved his team and gave us problems that we didn’t have in the first half. I also think there was some fatigue in some of my players. A few factors made the second half not as good as the first.
“We were intense without the ball and with the ball. Without the ball we pressed, we kept Mitrovic away from the goal. With the ball it was simple, one or two touches. We were really good.
“We needed it. The last three matches we didn’t lose, but didn’t win. Now we have two away matches, then two at home. Let’s see if we can get more points from those matches. The players have shown they can be a better team than they are.
“When there is solidarity in the team, everyone giving their maximum, you attract more positive things.”