By SABC Sport
29th July 2023
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".
Mainoo wrote on Instagram: "I'll be back stronger and in no time. Thanks for your messages of support."