I think it would depend on what sort of formation we were playing, and how much he was being actually asked to defend, what i mean is if we were playing 3 central defenders (out of our current squad say - Sagna (right hand side) Kompany (central) Mangala (left hand side) then it would mean the 2 wing backs would be given licence to get forward without having to be relied on to do too much defending as they will always be covered by the centre back from their side. If a move were to break down then there should be enough defenders to cover until the wing back could get back into position and help out. Essentially not too dissimilar to the job Alaba has done for Bayern on there left hand side on occasion. good on the ball, very athletic and fit enough to get up and down the pitch at a good speed.
the reason i think he would be good as more of a right midfielder / right wing back is because i think he could be very good at getting forward to support attacks from there, not necessarily the defensive side of things.
(I'm not comparing Alaba and Fernandinho by the way, only the role i think he could play)
Coughs *fernandinho on the right* coughs