I would label him as a second striker and i don't think he would give us full value as the furthest forward striker. I don't like the idea of him as a false 9 either.
Well second striker is still a striker. Plus he actually led the line for France in the Euros as the solitary striker and he wasn't a false 9.
There's always the possibility that we can play with 2 strikers which I think would suit both Martial and Rashford. Neither one of them is comfortable as an inverted winger (particularly Rashford) in my view.