The 21-year-old has enjoyed an impressive breakthrough campaign at Old Trafford and is in line to start Tuesday night's Champions League meeting with Sevilla.
United boss Mourinho says that McTominay, rewarded this week with a maiden call-up to the Scotland senior side after snubbing England, has more than made the most of his chance to shine following injuries to a number of key midfield men.
"He's playing well. He is using every minute on the pitch and outside the pitch to learn," he told reporters at Monday's press conference. "He's humble, he wants to learn. He doesn't want headlines or flashes. He just wants to work, improve and be useful for his team.
"He was progressively getting chances to play. He got the injuries of Michael Carrick, [Marouane] Fellaini, [Paul] Pogba and [Ander] Herrera - basically all of the midfield players I have. He was getting minutes and getting experience and the best way to improve is to be on the pitch. He is a good learner."
McTominay has started 11 games in all competitions this season, including the reverse meeting with Sevilla which finished goalless three weeks ago.