Mainoo was brought into the United first team and given game time by ex-boss Erik ten Hag.
He started in the FA Cup final last season, scoring in a 2-1 win over Manchester City.
Mainoo has reacted to United sacking Ten Hag and appointing Ruben Amorim in his place.
“We couldn’t go into the summer with nothing, just finishing where we finished in the table,” he told GQ, when asked about the FA Cup win.
“A trophy brings hope. But it definitely didn’t make the season a success (contrary to Ten Hag’s comments about winning a trophy).”
“I remember just the feeling around it was like, no one expects us to win,” he added.
“They’ve just written us off. I used that as motivation. I won’t say it made the season a success for us as a team, but we definitely needed it.”