Mainoo did not play a single minute at the World Cup in the summer and was frozen out by former United boss Ruben Amorim last season.
However, after the introduction of manager Michael Carrick in January, the midfielder has become crucial for the Red Devils who have relied on the 21 year old to guide the midfield alongside the likes of Bruno Fernandes and Youri Tielemans.
Mainoo should prove essential once more in the Manchester derby on Sunday afternoon in what is one of the biggest games of the campaign for both sides.
City arrive at Old Trafford as Premier League leaders after winning each of their opening three matches, while United have four points from nine in what has been a dodgy start.
Speaking on the game, Mainoo says he cannot wait as he continues to thrive under Carrick.
"It means a lot to me. I mean, coming up in the Academy, you play City twice a season and it’s always the biggest game. You know that being a fan, but where the coaches would big it up, it’s always been a big rivalry for us, and so yeah, I’m really excited.”
Mainoo continued: "Any time you beat City it’s positive and it’s big for the season, like it helps boost the team, so we’re not going to take that lightly, and we know it’s a big one. Hopefully we come out winners.”
On Carrick: "I’ve loved it. So much help, especially him being such an experienced midfielder and winning so much with this club. Anything that he tells you or any instruction that he gives you is gold, really. So it’s been good to learn under him and play for him.”
The 21-year-old has been outstanding in each of United’s matches so far this season and he should play a huge role against City once more in what should be an entertaining clash.