The 20-year-old midfielder has yet to start a Premier League match this season under Amorim, with his reduced role fuelling speculation over a potential January exit from Old Trafford.
Now, Keane has urged patience from the England international and insisted the responsibility lies with Mainoo to change his manager’s mind.
Speaking on the Stick to Football podcast, the former Manchester United captain, 54, said: “Are you telling me that he can’t sit there for another six months? 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.
“Sometimes a manager is on your case and what you have to do is prove them wrong. Jamie [Carragher] made a point that he never felt like a regular at Liverpool, I never felt like that at United.
“Every day is your chance to prove yourself to the manager and when the manager is picking his team, [he goes] ‘he has to play.’ He’s got to get that in his mindset.”
Mainoo enjoyed a breakout campaign last season after progressing through United’s academy, but has found opportunities limited following Amorim’s arrival.
The head coach has preferred the experience of Bruno Fernandes, 31, and Casemiro, 33, in central midfield, leaving Mainoo largely restricted to brief substitute appearances.
Keane also criticised the off-field attention surrounding the midfielder after his brother, Jordan Mainoo-Hames, wore a ‘Free Kobbie’ shirt in the crowd during United’s 4-4 draw with Bournemouth on Monday (15.12.25), in which Mainoo played 29 minutes.
Keane said: “And when he’s got his idiot of a brother doing all that, we shouldn’t even be giving that kind of stuff the time of day. Sometimes you’re just surrounded by idiots, especially the families.”
While Keane believes Mainoo should stay and fight for his place, other former United players have taken a different view.
Gary Neville has suggested Mainoo is between a rock and a hard place, arguing that Fernandes’ form makes it difficult for Mainoo to play in his favoured role and that a move elsewhere may accelerate his development.
Amorim has offered little clarity on Mainoo’s future.
Speaking ahead of United’s Premier League game with Bournemouth, the Red Devils manager said: “I don’t know what is going to be the future [for Mainoo]. Let’s see after this match.”
Amorim also referenced other young players, including 18-year-old full-back Harry Amass, when explaining his reluctance to rely heavily on emerging talents.
With the January transfer window approaching, Mainoo is expected to attract significant interest. Napoli have previously been linked, while Chelsea and Bayern Munich are also understood to be monitoring his situation, with reports suggesting as many as 12 clubs are tracking the midfielder.
