I think people are being overly precious about the idea of him playing 'on the wing'. The wide men in a 433 or 4231 can be any manner of things. With Ibra as the central striker, it makes a lot of sense to have two 'wide forwards', Rashford and Martial, on either side of him. He'll receive the ball and look to flick it through to their out-to-in runs.
If you watch a highlights reel of Rashford's senior games so far, the vast majority of his best moments came from a wide starting position anyway. Which isn't surprising, given that he's only recently converted to a central striker, and spent most of his development as a winger.
If we were talking about a Fergie-style 442 winger, clearly that would be a waste. But on the right of a 433 seems like the perfect position for him to me.