Rafa has said before that he played 4-2-3-1 at Valencia because of the players he had. He had two good wingers in Vicente and Rufete and an exceptional playmaker in Aimar, so he built the formation to get the most out of them. I think last season he thought Gerrard might be able to play similarly to Aimar, but he's probably realised that he's better when attacking from deep. I seem to remember something from Balague's book about this too, that because of the players we had he felt 4-5-1 would suit us better, think it was something to do with us not being able to hold the ball up.