Right now I'd start Rashford centrally together with Martial, with the added responsibility of occasionally drifting out wide right in order to provide width there.
--------Martial Rashford--------
James----------------------------
---Pogba---Fred---McTominay---
Shaw - Maguire - Lindelöf - AWB
--------------DDG-----------------
James and others will need to be rested/rotated at some point though and then the attackers will pick themselves.
Last season, Rashford managed 10 league goals in spite of playing 50 % of the season out of position in a highly dysfunctioning team and another 25 % of the season injured. He was absolutely flying when Ole came in, scoring top class vital goals like that one against Spurs.
He's got great pace, movement, dedication, work rate and many other attributes. Particularly, he's very good at stretching the opposition's defense and running onto through balls from the likes of Pogba. Yes, there's also a lot of room for improvement with regards to his box-play.
But just because he's been out of form in the last few games doesn't mean he should be written off as not good enough. Maybe he should be rotated, at the most.
I think Rashford can get 20 league goals in a season where the team is playing well as a whole and he is starting centrally, but even if he doesn't he's still a very useful player in many other ways. Does Firmino get 20 league goals a season for Liverpool? No, yet they won the CL with him as an integral part of their team. It's all about finding the best roles for each player and getting the most out of the squad as a whole.
Also, I think football might be moving away from the 30-40 goals a season strikers anyway. I think the most PL goals any player scored last season was 22. Rashford still got half of that despite the factors I mentioned above.