He basically doesn't fit into their system at all, if they adapt their system this season to a one upfront with early balls and pace then he will be back to his best. He may score a couple with Mata supplying, but they don't exactly light up the national side as a pairing, so can't really see much difference at Chelsea.
The other main contributing factor was that Torres and Gerrard had a telepathic understanding, very much like what Cole and Yorke used to have, he will do extremely well to find another midfielder to link up that well with again in the future.
His demeanor and attitude has also changed since moving to Chelsea, he knows he has made the wrong choice.
In Spain they had a name for him, I can't recall it off the top of my head, but it translated to "bad omen" or something similar, the Atleti fans called him this because they never won anything with him in the side, as soon as he moved they won the UEFA cup and Super cup. We never won anything with him in our side, and Chelsea have yet to win anything with him in their side.