I'm beginning to wonder if Skrtel has something of the "big-time Charlie" about him. In pressure games he can sometimes be immense - he's the only CB I ever saw outmuscle Drogba in his pomp, he was one player who did OK when we scraped past Cardiff in the Carling Cup Final and he won his battle with Costa when Slovakia beat Spain recently.
Agger did that once as well (though I think he also had at least one game where Drogba dominated him). I think Skrtel is capable of winning the individual duals, but his overall concentration/awareness is not good enough. The way we play risks defenders getting isolated so you need centre backs who are switched on to everything all the time. I don't get the sense that he is.