Yeah, you are very much right on your take here, it's very true that Amorin is a very talented manager no doubt, but managing a team like Manchester United comes with his own added pressure and troubles, and with the quality of players at Manchester United right now, expecting a league title from them now will be too much to ask, because they lack the quality and consistency to mount a title challenge, but a place in the top four is very much achievable unlike winning the league title right now.
Amorin is a very decent manager but he hasn't proven himself in the top league as the premier league or other major league. He did well against Manchester city but that is as Manchester city aren't in form. They have lost three matches so far including the game against sporting. At Manchester united, Amorin is going to face a completely different challenge than what he was facing at his old club as he would no longer be the underdog but now the big teams, he'll be facing everything team with the mindset of a big club and this might make him to make mistakes that his opponents will use to win him. At sporting he can take team unaware but at Manchester United, all teams will be ready for him.