Mainoo pushed to leave United last month, but management blocked his departure.
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However, Amorim admits the midfielder hasn't done enough to warrant a starting place.
"Like a lot of guys, he wants to play more," said Amorim. "I didn't have a conversation with him before the window closed, but I did this week. Because I don't want Kobbie Mainoo thinking I was having any conversation with him just to hold onto him.
"I believe a lot in Kobbie, but some of you think that Kobbie Mainoo is already done. I think he can do so much better, he can improve in a lot.
"I think for some guys it is enough, but for him it is not enough. Maybe it's not fair, but I think I'm helping Kobbie Mainoo and that's it. He will have opportunities like the other guys."