I guess you could teach others the role, but he plays DM, CB and RWB in one game for us - it's not so much the physical/technical ability to do that, it's the tactical understanding of when to do itI like Walker, but I think if you can find a very fast, pretty physical RB with decent technical skills, they could do his role.
He does have very obvious flaws in the final third but he's one of the best defenders in the world when defending half the pitch with a high line.