The 19-year-old has seen himself emerge as the next great hope in midfield for the Red Devils.
Mainoo, who helped the team win the FA Cup last season, is set to renew his deal with a huge wage rise.
Sky Sports states that United chiefs are working hard behind the scenes to get a deal done.
Manager Ten Hag stated recently: "I think he is rewarded by us as a club, as a team. We are so pleased with his development and for us and for him is to keep this process going.
"The leadership in the club has to talk about this and has to make a decision about this - of course we do that in co-operation.
"That is the thought - we will always have to discuss our players that are progressing well and what we will do with them. And when we have news, we will tell you first."