Pereira, who joined Man United in 2012, made his first-team debut for the English giants in the FA Cup against Championship side Wigan Athletic on January 29.
The 20-year-old spent the first half of this season on loan with Portuguese side Belenenses, but was recalled in January as Mourinho planned to introduce the stopper into the first-team set-up.
Pereira is currently on the sidelines with a knee injury, but Mourinho has suggested that he has big plans for his fellow countryman.
"We have a third goalkeeper that I think will be the best Portuguese goalkeeper of the next generation," Mourinho told Man United's official website.
Pereira has made four appearances for Portugal's Under-21 team. body check tags ::