Amorim was new and didn't understand much about the Premier League clubs which made him feel that, he can't make it to the top four because he wanted an automatic qualification to UCL. That was his target but Tottenham shattered that dreams. Therefore, I wouldn't blame him for United misfortune last season. You don't need to underestimate Amorim until, we see his efforts at the end of this upcoming season.
You're right, we shouldn't underestimate Amorim at this point. He's only been coaching Manchester United for one season, so it's understandable that he's still struggling to adapt. Coaching big teams in the Premier League isn't easy, as they always boast a wealth of star players, who are undoubtedly challenging to manage. Manchester United is no exception. Therefore, we shouldn't underestimate Amorim, as I believe he'll show his true potential next season. If Manchester United's performance under Amorim continues to be poor next season, we can conclude that Amorim is simply not suited to coaching Manchester United and should be sacked immediately.
Typically, a coach's success is determined by their second season. By their second season, a coach should have adapted to all the factors and should be able to demonstrate significant change in the team they coach. However, if a football coach can't do that, or hasn't been able to improve his team, it can be concluded that the coach is simply not suited to coaching that team. Therefore, we shouldn't criticize Amorim too much right now, because he could very well lead Manchester United to a comeback next season. Because, even though Manchester United hasn't been aggressive in bringing in players at the moment, I'm sure Amorim has a pretty solid plan for next season.