The talented 19-year-old scored the winning goal in the FA Cup final against Manchester City.
United beat their rivals 2-1, with Alejandro Garnacho and Mainoo scoring their goals.
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Per The Telegraph, Mainoo will have to wait for a new and improved contract for a few months.
United will be focusing on incoming and outgoing transfers, before they look at contract renewals.
Mainoo is valued at the club and they will certainly increase his salary with the next contract.