The 20-year-old asked to go out on loan during the summer transfer window in pursuit of regular game time, but had the request quickly denied by Man United.
Mainoo has since played just 138 Premier League minutes under Ruben Amorim this season and is said to be just as frustrated.
Now, according to transfer guru Fabrizio Romano, struggling West Ham are hoping to sign the midfielder on loan in January.
Speaking on his daily podcast, Romano explained: "Mainoo wanted to go on loan late in the summer transfer window, but Manchester United and Ruben Amorim decided against it at the time.
“Mainoo is not playing regularly, so he wants a loan move. There is interest from West Ham, offering guaranteed minutes."
Nuno Espirito Santo is said to be desperate to sign a ball carrying midfielder but will face competition from Serie A champions Napoli.