His positioning is very very good, but his weakness in the air and lack of speed mean that against teams that have fast skillful strikers (ie most of the top teams in europe) he can't hold his own any more. While Skrtel and Agger are both behind him mentally, the fact that both of them can defend very well, are goal threats in the air, and are faster, mean that they are more suited towards the way the EPL is played. Also, in Agger we have one of the best ball-playing central defenders in the world. he can pass, dribble, and shoot as well as most midfielders! frankly, he should be our most important defensive asset for the next 5 years at least. To be honest actually, while I would prefer Skrtel playing in most games, I think the most important issue is that Agger is a starter next year