The England midfielder is pushing for a departure after a lack of minutes under Amorim since the Portuguese's arrival.
Amorim has been criticised over his treatment of the academy graduate and ahead of today's clash with Aston Villa, he made a challenge to the player.
Amorim said: “Kobbie needs to fight for his job. It is not a bad thing to be on the Manchester United bench at 20.
“I remember Ronaldo on the bench, Rooney was on the bench sometimes. Juan Sebastian Veron wasn’t playing. I remember all the United history.
“I’m not trying to say to Kobbie, ‘You are a big player here, let’s put him on the bench to show something.’
“No, no. What I’m saying is the opposite. I just want to help the team to win. And to help Kobbie be a better player.”
Amorim has also urged Mainoo to speak to him about his frustrations rather than through social media.
He added: “Kobbie is a shy boy but he’s responsible for his actions. He played well last game, I’m focused on that.”