He has moved into the club's senior changing areas at Carrington, over the past couple of weeks.
Even though Mainoo has been a first teamer since pre-season, The Mail states he was getting changed with the youth players.
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The talented teenager is likely to discuss a contract extension with a pay rise in the summer.
Mainoo has become an automatic pick in midfield for United next to Bruno Fernandes and Casemiro.
After Donny van de Beek and Facundo Pellistri left United in the winter, Mainoo now has a first team spot in the dressing room.