The midfielder is struggling to play three games a week for United.
But Amorim says: "The players will get used to that. I'm trying to be honest with you guys. I talk openly about the game. You are focused on what I said about Ugarte.
"Ugarte was tired because he did a great job against Liverpool. If you look at the game against Arsenal, maybe Ugarte and Bruno (Fernandes) were the best players against Liverpool. They didn't play against Newcastle. I think that happens with all players in the Premier League."
Amorim did, however, admit he made a mistake in terms of his selection when putting the tired Ugarte up against Southampton’s energetic midfield.
“Also, it is my fault because if you lose the timing of the pressure, if I don’t explain it well, the pressure is always lost in that, and you feel it today,” he added.
“Mateus (Fernandes) was, with the number seven (Joe Aribo), creating a lot of problems for Manu because Manu was running against two guys. So it’s more my fault than his fault. He is going to get used to playing every three days.”