A highly-regarded talent at Old Trafford was urged to stay put last summer as the Red Devils pieced together their plans for 2017-18.
The 22-year-old was, however, of the opinion that his development would be better served by more regular football in Spain, having spent the previous campaign with Granada.
Pereira admits that was a tough decision and insists he remains fully committed to United long-term.
He told The Guardian: “[At first] I was thinking that I want to stay. But as the pre-season went on I had the feeling: ‘I don’t think I’m going to play enough, I’m not going to get a lot of minutes. I have to play, I have to continue developing’.
“I knew I would get some game time at United but here I’m playing every week and being important for the team.
“It was a very difficult decision. I never want to leave United but I want to play as well. In the end it will make me a better player and I will still be a United player.”
Pereira also believes that Mourinho will forgive him for jumping ship, with United thinking enough of him to have tied him to a new contract through to 2019 shortly after his departure for La Liga.
“He wanted me to stay there and it was important [to him], but I needed to do this. This was important for me and my career. And I’m sure if it works out, next year I will be back there and it will be better for both of us.
“In the end I think he is happy because he is a very honest coach. I was talking to him right up to the very last minute when I came here.”
Pereira has made 22 appearances for Valencia in all competitions this season, netting one goal and providing three assists.