Rashford was again left out of the United squad for the 1-0 win over Fulham, a game in which Amorim's side scored the only goal thanks to Lisandro Martinez's deflected shot.
Marcus Rashford has not been named in United's Premier League squad for nearly two months; Amorim said Rashford does not have the attitude of someone who "gives everything every day".
The January 2025 transfer window is about to end, Amorim said, if Marcus Rashford does not change himself, he will not change the way he uses personnel.
When asked about the reason for not using Rashford, Amorim said: “There is always a reason, the training process, the way I see a football player should do in training, as well as in life. It happens every day, in every little detail.
So if things don't change, I won't change. It's the same for every player, if you do everything right, give your all, we can use every player.
People can see on the bench we lack a bit of pace. But I would put Man United goalkeeping coach Jorge Vital on the bench, rather than a player who doesn't give his all every day," Ruben Amorim criticized Rashford.
Amorim, the Portuguese coach who arrived from Sporting CP in November 2024, will celebrate his 40th birthday on January 27.
But after the win over Fulham, he said he felt much older after his short time at Man United.
"Not 40... I'm 50. After two months at Man United, I feel like I'm 50. It's an honour to celebrate my 40th birthday here.
The feeling of winning and the three points was really important for us. You can't see a big improvement in the team and that's true, but the win helped us a lot.
We were unlucky in some games. Today was not the best game but Man United won. The important thing is that I have a clear idea of what I want to do," Amorim said.
Amorim also expressed concerns over the future of Alejandro Garnacho, who played against Fulham, but was taken off after Martinez's decisive goal.
Garnacho is a target for Napoli and Chelsea, with United likely to sell the academy graduate for a "pure profit" to help them comply with the Profitability and Sustainability Rule (PSR).
"Nobody knows. Anything can happen. Garnacho is improving in every aspect of the game. I'm trying to find the best position for him, today he played a little more open, not playing in the central position as much. We need players who know how to play one-on-one but I don't know what will happen," concluded the Man United coach.