The midfielder was pushed out the squad by former manager Ruben Amorim at the start of this season but has since had a revolution under interim boss Michael Carrick after he was sacked earlier this month.
Mainoo has 18 months remaining on his Manchester United contract, plus the option of a further year and the club are set to offer him a new deal as he starts to prove his worth in midfield.
Speaking to BetGoat, Hamann has spoken on the England international who he feels would slide into any top Premier League side with ease.
"I was very surprised with the Kobbie Mainoo situation because when I saw him break through at Man United and then in the Euros, it was just a breath of fresh air. A very clever player. He also scored a couple of good goals - I remember one where he beat two players and curled it into the far corner. He is an excellent player.
"To not find a place for such a young kid at Manchester United - with the struggles they have at the moment - really flabbergasted me. If he did decide to move away from Old Trafford, he would be good enough to start for Arsenal, Chelsea, Liverpool or Man City.
"But even if he doesn't start every game, he should be in and around the first team. He will be one of the first 13 or 14 players - probably the first option off the bench. Not to play a role for Man United, with how bad they've been for the last 18 or 24 months, really did surprise me. I hope that he's able to benefit under Michael Carrick now."
With the soon-to-be-34-year-old Casemiro departing at the end of this season, United need to hang on to Mainoo who was keen on a move to Napoli in the summer.
United return to action on Sunday as Carrick’s side face title favourites Arsenal at the Emirates in a clash where fans will be keen to see more of Mainoo who looks to be the future of the Red Devils’ midfield.