What I see, Amorim actually only needs to do a simple in-depth study and what is clearly visible is the back line in Man United's defense which still looks very weak. Even everyone would say the same thing if Man United's forward position was still a little better and could still be relied on. However, the defense always appears inconsistent. So, Amorim just needs to start everything from the worse things like replacing defenders who are no longer productive. If the coach can do this slowly, I am sure Man United will definitely experience slow improvements.
Manchester United doesn't have a quality squad considering their name to be honest. There are really weak areas. The defense is already weak as you said and I don't think the offense is good either.
To get over these problems they need to make new signings this month. If they want success this season then they should let Amorim bring better names for the areas in immediate need. Amorim likes to play with a formation including 3 defenders as you know. He needs trustworthy players for this system to work smoothly. As far as I have heard they are trying to convince Napoli into a swap deal between Osimhen and Rashford as well. If they make the needed transfers now I think they can increase their chance for the Europa League title.