McTominay has explained he owed Mourinho a "tremendous" amount of credit for believing in him.
Speaking to Sky Sports, he said: "I obviously don't want to speak too much about previous managers but Jose's very special to me. He still keeps in contact with me now and keeps in touch with me after some games.
"He'll always have a special place in mine and my family's heart because he was the one who brought you in, he was the one who trusted you, he's the one who had belief in you.
"I owe a tremendous amount of credit to him because if it wasn't for him I might not necessarily be sat in this seat."
The Scottish international believes that remaining focused on the present is the key to success.
Questioned about how he had felt in the build-up to his debut, McTominay added: "It just came around so quickly, you don't even have time to think about it. And then when you do you have to take it, and if you don't then you'll probably be somewhere else.
"I think you've just got to take life and football as it comes. You can't think too far ahead, you can't cast your mind back to three years ago because circumstances change."