Pereira has been at United since youth level.
The Brazilian went against Mourinho's advice last season by moving to Valencia, where he played 29 times.
Andreas Pereira insists his two-year loan spell has totally helped him becomes a better player and has changed him as a person.
The 22-year-old midfielder started in Manchester United’s opening-day 2-1 win over Leicester in the Premier League last Friday, having spent a year on loan at Granada and then Valencia in La Liga.
However, while speaking with Sky Sports News, Pereira said: "I learned a lot of things being on my own, I lived on my own there, and it's different than living over here.
"I'm more professional now, I know what my body needs every day, I'm more mature now than I was two years ago. They've been really good for me, I think I've totally changed as a player and a human being after these two years.
"It was very difficult for me to take that decision. Like Mourinho is one of the best coaches in the world, and he tells you to stay, and if you go against it you've only got things to lose in your head really.
"I was nervous about that, but it came out well. We had a great year in Valencia, and I am a better player, and this season he has a better Andreas than he could have had last year.
"He hasn't told me [that I have improved], but I think he will see this in games. I have a very good relationship with him, he is very good with me, very honest."