Alerted to news that Mainoo had made a transfer request at United last Thursday, with the midfielder wanting to move away on-loan.
The Daily Mail says three clubs in the Premier League - Tottenham, Aston Villa and Everton - were all in contact. While from the continent, Marseille, Roma and Napoli also made approaches.
However, United insisted Mainoo was a required player and that he was unavailable for loan on Monday.
Before the win against Burnley, United manager Ruben Amorim said: "I want Kobbie to stay.
"He needs to fight for his place, and we need Kobbie, so that is not going to change."