Pereira and nine other members of Nicky Butt's reserve team have been drafted in for the meeting with Crystal Palace to keep his first-team players fresh for the Europa League final next Wednesday.
Mourinho has described Pereira as the "best Portuguese goalkeeper of the next generation" ahead of what could be the 20-year-old's first start for the Old Trafford side.
In response, Pereira told Sky Sports News: "They're nice words. Now I need to prove that he's right.
"When one of the best managers in the world puts confidence like that in you, you can only feel good. I want to pay him back by training and performing well.
"I'm working hard every day and the next step is on Sunday, so let's focus on that and we will see. It's a big day coming up for the young players. As the manager said, some of us will get a chance and we're looking forward to it. Hopefully we'll get a good result, everybody will enjoy it and we'll play well."
Sergio Romero and David de Gea will both be rested ahead of the clash with Ajax in Stockholm next week, with another youngster, Kieran O'Hara, competing with the Portugal Under-21 stopper for a starting spot on Sunday.