Liverpool were actually quite strong defensively in the middle part of last season, when they had the centre three of Can, Sakho and Skrtel. It's when they tried to change that formation that they started to ship goals. I suspect Can might end up playing quite a bit in central defense next season, even though it's not his best position. Wisdom and Ilori might also play, but it's hard to judge how they would do. I agree that they should buy a really high quality central defender, but it's harder to say who that would be.