I'm not entirely sure that's true tbh
If Diaz comes, he plays on the left. That's where Richarlison currently plays and clearly we aren't dropping him, so you'd assume he will be playing just off the front man in a sort of 4411 system (not a million miles away from a 442). Effectively it does replace James, just we will be playing more direct. If we need a number 10 then Iwobi and Gray can both play there.
I'd have liked to have seen a like-for-like replacement for James, but think the change in style this season may mean that we don't necessarily need one