The 20-year-old has been with the Red Devils since 2011, but has only made 11 first-team appearances for the club and just four of those have come in the Premier League.
Pereira was given the second half of Saturday's pre-season clash with Wigan Athletic at the DW Stadium, however, and the midfielder scored his team's second in a comfortable 2-0 win.
"Andreas has lots of talent. He has incredible vision and an incredible technique," Mourinho told MUTV.
Pereira, who scored once in nine first-team appearances last season, spent six years in the PSV Eindhoven youth system before joining Man United at the age of 15.