It shouldn't be that big of a deal defensively, at least not with our roster.
Unlike in the NBA, most college offenses aren't designed to take advantage of that type of a matchup problem. You rarely see a bigger guard take a smaller guard into the post. Obviously it's easier to shoot over a smaller defender, but it's not like any of our taller wing players are able to contest many shots anyways. If Simpson is better than our current players at stopping penetration and chasing his man around screens, I don't think his lack of size would be a huge detriment.
If Simpson doesn't play much, it's probably for the reasons MattD said--that he isn't yet college-ready.