Mainoo's problem is that Amorim sees him in competition with skipper Bruno Fernandes, who has not been replaced since the win at Liverpool on 19 October six games ago.
The 20-year-old could have joined Napoli on deadline day in the summer, having told United he wanted a loan move in order to try and preserve his place in England's World Cup squad.
That plan was blocked but it is understood Mainoo's opinion remains the same and he feels he needs a loan move to help develop his career.
Amorim has shown signs of frustration at the regular questions about a player who in 2024 scored in the FA Cup final victory over Manchester City and followed up by starting the Euro 2024 final with England.
Asked if he understood why Mainoo's status as a homegrown English player made him of specific interest, Amorim said: "Of course I understand, and my job is to answer.
"But you always ask me the same thing. I understand what you are saying. You love Kobbie. He starts for England. But that doesn't mean that I need to put Kobbie [in] when I feel that I shouldn't put Kobbie [in]. It's my decision."
Amorim offered no guarantees the situation will change even when Bryan Mbeumo and Amad Diallo leave for Africa Cup of Nations duty with Cameroon and Ivory Coast later this month.
"I don't know, I don't know," he said.
"It's the same question. I don't know what is going to happen. It depends. I've seen the training, if it's the best thing for the team, I will put [him in], that is the only way I know how to respond to that."
Amorim has opted not to give Lacey his senior debut even though United have failed to win two of the three games he was on the bench for and the Liverpool-born player was the only attacking option with Benjamin Sesko and Matheus Cunha both missing games through injury.
The lack of involvement gives the impression Amorim doesn't trust the academy to develop players to the standard he needs.
"Any players?" he said. "We have Kobbie Mainoo…"
When asked specifically about Lacey, he added: "So you are asking me why I don't put Lacey (on) to play, when I have other guys that are internationals who are also on the bench.
"I try to put (on) the best guys, the prepared guys, for this moment of the club, in trying to win the game. That's the only thing."