Mainoo is weighing up his future at Old Trafford after being publicly told by manager Ruben Amorim his only chance of a regular start would be by being selected ahead of captain Bruno Fernandes.
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The Daily Mail says Real Madrid and Atletico Madrid have both been alerted to Mainoo's frustration.
As many as ten clubs are already in contact with Mainoo's agents, with the major based in the Premier League.
Chelsea were keen towards the end of last season, but have since pulled back as they focus on strengthening other positions.
For the moment, it's yet to emerge whether United are prepared to sell the England midfielder outright, or send him away on-loan.