Out-of-favour Rashford has not played for his hometown club since December 12 and appears to be firmly out in the cold under the Portuguese after again not making the squad for their 1-0 win at Fulham.
United manager Amorim went as far as saying that 63-year-old goalkeeper coach Jorge Vital is more likely to gain minutes than the England forward.
"It's always the same reason (why Rashford is not playing)," Amorim said.
"The reason is the training, what I think a footballer should do in training, in life and every day. If things don't change, I will not change.
"It's the same situation for every player. If you do the maximum and the right things then we can use every player.
"Today on the bench we missed a bit of pace to change the game but I prefer it like this, I will put (in) Vital before I put (in) a player who doesn't give the maximum every day, so I will not change in that department."
Ruben Amorim shouts instructions during Sunday's 1-0 win over Fulham. (AP PHOTO)
However, Rashford took to social media later on Sunday evening to congratulate his team-mates for their success in west London.
"Congratulations on the win lads," he posted on his Instagram stories.
Lisandro Martinez's 78th-minute winner helped United to their second Premier League victory in three matches. The defender scored with a long-range deflected effort which looped over Cottagers goalkeeper Bernd Leno.
The win moves United up to 12th and Amorim continued: "It's a good day for us, it was not a great performance but we fought for the win."
