It's not the wildest suggestion in the world, I don't think. We've played O'Shea, Jones, TFM, and Axel there anyway and they did Ok.
I for one won't be against trying it out. He's not the most aggressive CB but I reckon he is a better tackler than both members of McFred. In terms of defensive positioning, he'll probably be a lot better considering he knows what protection a CB would appreciate, and we know this isn't a strong suit for McFred.
One area I'd be concerned with is whether he can pass well under pressure, and just as importantly, trust himself to do so. He does seem comfortable on the ball but controlling the ball in the thick of it in midfield is very different to passing it to Maguire with no opponents around you.