Skrtel's shit positioning creates the space, Lucas failing to track the man into that space further exacerbates the situation. Everything Ryan said.
there's a fair amount of confusion over Gerrard's role though, he tucks in throughout the game as more or less a third centre halve, so when on the defensive why is it so difficult to adjust? There's a gaping hole there with Gerrard stood a couple of yards ahead of it, when ideally he should be dropping into the space, in the same way he does when collecting possession. He just strolls, Skrtel fails to recognise it early enough and there's acres of space then for the player to run into.
The centre backs play wide apart throughout matches, it doesn't take much more than a direct pass into that area for it to be exposed quickly. Most teams do us that way, they don't do it by intricate passing play, they do it by hitting us direct and early before we have a chance to regroup/position ourselves.