He's great if he actually gets to the ball but his positioning is woeful. Constantly out of position or switched off, but uses his pace and strength to recover. When he was at Celtic it was even more obvious because he was half asleep, just bullying out of shape Scottish players, then getting horrendously exposed whenever they played in Europe.
I can see why he wasn't hugely chased by teams as he had very much the Jean-Alain Boumsong look about him in Scotland, but he has at least done a better job of staying switched on at Southampton, but there system helps a lot with that. Everyone looks good in their defence