The 18-year-old midfielder, who made his first-team breakthrough last season and was playing a prominent role on the summer tour of the United States, was injured in Thursday’s friendly against Real Madrid in Houston.
Mainoo has flown back to Manchester for further assessment but a report on the club’s website said he was “expected to be ruled out for the first part of the 2023-24 season”.
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Mainoo wrote on Instagram: “I’ll be back stronger and in no time. Thanks for your messages of support.”
United continue to be linked with a move for Fiorentina midfielder Sofyan Amrabat as Erik ten Hag bids to strengthen his squad further during what remains of the summer transfer window.
They have already signed Mason Mount and Andre Onana and have reportedly made a verbal offer to Atalanta for forward Rasmus Hojlund.
