The teenager is set to sign his third new contract in the past three years at United.
Per The Mail, United want to ensure there is no temptation to leave on Mainoo's part.
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The club will reward him with a wage rise, as he is now a key member of the first team.
He has played 25 games for the club this season and may end up with more than 30 appearances before the season is done.
He would have been playing even more often if not for an early season ankle injury.