I don't immediately see a role for Rashford in a 343. He's not a hold up number 9 and his creative interplay around the edge of the box was pretty average even when he was on fire for goals. A wingback he is not. Its hard to see a future for him, especially when his form is in the pits.
That said, Rashford is such an iconic character in Manchester that it would feel incredibly odd for him to leave. Not only is he on the back of a tonne of United shirts, but he's like the only footballer your nan can name. Id be gutted if it didnt work out. But we can't carry him forever based on how profile he is, especially if he doesnt suit the new manager.