United won thanks to a late Lisandro Martinez strike, with Amorim again leaving Rashford out of his matchday squad on Sunday.
Asked about the situation, the manager said: "It's always the same reason - the training, the way I see a footballer should do in life. It's every day, every detail.
"If things don't change, I will not change. It's the same situation for every player, if you do the maximum and right things we can use every player.
"You can see on the bench we miss a bit of pace on the bench, but I would put (goalkeeper coach Jorge) Vital before a player who doesn't give the maximum every day."
On the win, the manager said: "Today was not the best match but we managed to win.
"The important thing is I have a clear idea of what I want to do. Sometimes I'm frustrated.
"We want to play a different style and we are going to play different style in the future."