I'm sure we will see a lot of trial and error this season, especially in that attacking trio. Rashford isn't a perfect fit on paper for anywhere in Amorim's system, but at least in the number 9 spot he has the maximum chance of scoring goals. There is zero chance that we let Rashford go next summer without having given him a fair chance to fit the system.
From what I understand of Amorim, he doesn't want his strikers hounding the opponents at every turn early era Klopp-style, but rather is more focussed on getting them to close off passing lanes so that the opponents pass into central areas, which is where they press from. While I wouldn't say that closing passing lanes has been an obvious strength from Rashford either, its still less demanding than trying to get the initial turnover. It also obviates the risk from Rashford's tendency to switch off when tracking back opponents. And one thing he at least offer is running onto balls in the inside left and right channels from a central position.
The negative commentary from fans about this is disappointing. I personally don't have much faith in Rashford coming good, but that doesn't mean I don't want Amorim to try and make it work, and I certainly don't think its a negative that he's trying. There's a lot of under-performers in this squad, we can't just ditch them all. We need a manager that gets the most out of his players. I would hate it if a manager came in with preconceived ideas that certain players are shit and beyond redemption.