Carragher and Skrtel are good centre backs, Kyrgiakos is probably ok too. Agger should be a very good one, but he seems to be constantly injured. Never really rated Skrtel that high anyway, but Carra was obviously one of the best CB's in the world for a few seasons. Anyway, I don't think it's enough to have one very good (injury prone) CB, 2 good ones and 1 ok for a side that has aims to win the league and/or the CL.