The 21-year-old, who came through the ranks at Old Trafford, made his first Premier League start of the season in the 1-0 win over Bournemouth on Wednesday.
Mourinho heaped praise on McTominay following his performance against the Cherries and insisted that the academy product is no longer an up-and-coming player.
"I didn't realise that he was young Scott," the Portuguese told the Manchester Evening News. "For me he is not young Scott anymore - he is just Scott, a player from the first team squad.
"Scott played this season 90 minutes in three different competitions, played against Swansea in the cup, tonight and Benfica and for the three times he played 90 minutes, not 87, 89 or 85. Why? Because in every match he was playing really, really well."
McTominay, who has made six senior appearances this season, is contracted to United until June 2021.