The fact is he wasn't, I've been through the last 21 games of the season and of those Carra played 18, 13 at CB and only 5 at RB, 3 of those were in the CL and the 2 in the league were against Bolton away and the bitters at home. I think the reason he was played at RB wasn't a case of there being a better CB pairing but more probably because Rafa was concerned about the RB position.