The England striker has not played for United since last month, after he admitted he was ready for a new challenge in his career. Amorim said on Sunday he would rather put a goalkeeper coach on the bench than a player not giving their all.
"Imagine a talent like Rashford, our team should be so much better with him. But he has to change," Amorim told reporters.
"If he changes, we are more than welcome (to include him), and we need it. But in this moment we have to set some standards. We are waiting for Rashford, if he wants it really bad."
Amorim said he had nothing personal against Rashford, who has made 287 Premier League appearances for United since 2015.
"I have to make the same rules for everybody," the manager said.
"We are a better team with Rashford, that is clear, but until the right moment I will not change my mind."
A draw on Thursday would be enough for United, fourth in the standings, to ensure direct qualification to the round of 16 and give their disappointing season a lift.
"I think it is clear we can improve as a team, all of us, if we have time to train and if we put some wins together. So, it will be so much easier," Amorim said.
