Mainoo, who scored in the 2024 FA Cup final and started for England in the Euro 2024 final, requested a loan move in August due to a lack of playing time. United rejected the move, and the midfielder has since made just one Premier League appearance this season - a second-half cameo in the 3-2 win over Burnley.
Amorim said he delayed speaking to Mainoo until after the transfer window to avoid giving the impression the club was only trying to keep him.
"I believe in him a lot," said Amorim. "But some people think he's already the finished article. I think he can do so much better. For some players, talent is enough - for him, it's not."
Mainoo has been competing with captain Bruno Fernandes for the number eight role but has yet to fully meet the tactical demands of Amorim's system.
"If he plays as an eight, he has to reach the box, track back, and cover space," Amorim said. "As a six, he sometimes moves too quickly after passing - he's not a reference point."
Despite limited minutes, the United boss confirmed Mainoo will continue to get opportunities.
United face Manchester City in the Premier League on Sunday.