It's better to take a decision early than waste a whole lot of time and still sack Amorin at the end, same thing that they did with Eric Ten Hang when he was man Utd manager, Manchester united didn't sack Eric Ten Hang when they should have sacked him, now they're using the same approach with Amorin who under his management, united is showing no sign of recovery, and this is why I like the management team of Chelsea football club, they don't waste time to take action, if they see the coach isn't performing to their expectations.
I agree that things are not looking great for Man United, but since they let Amorim carry on for another season and allow him to do all the shuffling to the squad, it wouldn't make sense to sack him now. He should be given a reasonable time to make the new team work.
If Leverkusen sacked Ten Hag because of the results of the first two games, that would also be unprofessional, but apparently, the main reason was his failure to get along with the players, who didn't trust him and the changes he was making. If a manager gets conflicted with the squad at the very beginning of the season, it's clear that nothing good will come out of it, so letting him go was justified for that reason.