That's according to former United scout Mick Brown.
Mainoo continues to be linked with a move to Serie A after a difficult season so far.
But Brown told Football Insider: “They know they made a mistake letting (Scott) McTominay leave.
“I was aghast at the fact they were even contemplating letting him go given the issues they had in midfield, and he’s done an incredible job since leaving. He’s gone away, knuckled down and become an absolute superstar for Napoli and for Scotland.
“But I think Man United have learned from the mistake of letting him go, and they’re going to take a different approach towards Kobbie Mainoo.
“There’s been a lot of talk about his future, will he stay, will he go, will the manager give him more minutes or is he going to stay on the fringes?
“As far as the club are concerned, there are no plans to let him go, because they know if he left it would be the same situation we’ve seen with McTominay. He would go and impress wherever he went, and all of a sudden everybody would be asking why he wasn’t allowed to do that at Man United.
“So if those offers do come in for him, I’d be very surprised if they allowed him to leave, and I still think he’s a huge part of their plans for the future.”