Mainoo is being monitored by Real Madrid and Atletico Madrid as he remains on the outer of manager Ruben Amorim's plans.
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The Sun says the midfielder would be ready to hit the ground running should he make the move to Madrid.
Mainoo has been learning Spanish since the arrivals at United of Amad Diallo and former teammate Alejandro Garnacho, now of Chelsea.
Both youngsters speak Spanish and Mainoo wanted to learn the language when youth teammates to understand them better.