United won the Europa League tie on Thursday 3-2 on a tense night at Old Trafford.
Amorin said afterwards: "I think we did four substitutions because of the fitness of the players, not thinking about what is best for the game.
"We are in that moment. We need the squad all together fit, and you have to take some risks and that's it."
Fullbacks Tyrell Malacia and Luke Shaw both saw minutes on the night.
"We improve on the quality with the ball. We had more chances (than the Ipswich game). We were more dangerous, but we will have to change the players," the boss admitted to TNT. "You can feel it. We try to press all the time.
"I think in the last 10 minutes was tough and it's different for the players. I felt Rasmus (Hojlund) was dead. I felt that some players were really, really tired. Antony came from injury. He had to go out of the game."
- Get breaking football news faster with the new Tribal Football app! Download now on iOS and Android to stay ahead of all the latest transfers and football updates. Download now: App Store, Google Play