Ruben Amorim has consistently left out Mainoo this season, with the 20-year-old academy product linked with a move away in January.
During the 4-4 draw against Bournemouth on Monday -- where Mainoo again came off the bench -- his half-brother, former Love Island contestant Jordan Mainoo-Hames, was pictured wearing a T-shirt printed with the message "Free Kobbie Mainoo."
"When he's got his idiot brother doing all that stuff -- we shouldn't even be giving his brother the time of day," former United midfielder, now a Sky Sports pundit, Keane insisted on Stick to Football podcast. "Because sometimes you're just surrounded by idiots, especially in families.
"You come in after the match, and your brother did that ... if my brother even done that, you would be looking at him going, 'what are you doing?' You would, and do you know what -- do you think his brother's done that without asking him? He's under contract, probably been paid a decent wage."
Mainoo is yet to start a Premier League game this season.
He is keen to find more regular games in an attempt to force his way into England's World Cup squad, and has been linked with a move to Napoli.
"He's 20 years of age, what's wrong with sitting and learning your trade? And even if he's not getting a chance, we've all had to do it," Keane added.
- Bruno Fernandes: Portugal not better without Cristiano Ronaldo
- Man City monitor Antoine Semenyo amid £65m clause - sources
- Mainoo's brother wears 'Free Kobbie' shirt at OT
"Sometimes a manager is on your case and what you have to do is prove them wrong. Every day is your chance to prove yourself to the manager. He's got to get that in his mindset.
"The life of a footballer is about trying to prove people wrong. I have no problem players going out on loan, it can work for everybody, but sometimes as a player, your biggest challenge is here at Man United and getting into that first-team.
"Even if the manager is saying 'you're not going to get in over Bruno,' prove them wrong!"
