The Red Devils were beaten 2-0 by the Gunners in the Premier League on Wednesday night.
Despite the away loss, which came through two set piece goals, Amorim felt that rotation was necessary.
"It’s not because I like to change all the time but we have to have all the squad fit," he explained.
"So, for example, Harry Maguire has a time limit, Tyrell Malacia has a time limit, Mason Mount has a time limit, Leny Yoro has a really time limit.
“So we have to manage winning games, different methodology, different way of playing, more metres to press and tomorrow we have to assess and the guys that are a little bit at risk to some injury will not play.
"So we have to have everybody to work, to play and to improve the squad."
- 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