Mainoo, a Manchester United and England youngster, burst onto the scene last season.
The Red Devils are hoping to tie him down to a new contract but may sell him if he continues to demand over £200,000 a week.
German giants Bayern and Premier League rivals Chelsea are the two most interested parties.If United do have to part with Mainoo, they would much prefer to sell him abroad.
United coach Ruben Amorim challenged Mainoo to show he deserves to be a top earner at the club.
