There's no one to blame for Song's inept defending other than Song himself. If he can't cut it defensively when he's not sitting comfortably in front of the back, without really having to do much thinking or intelligent moving/pressing - then he's a poor player and not worth persisting with. He's been asked to play with a little more dynamism and energy, that's all. We're not asking him to re-invent the wheel - he simply doesn't have the game understanding to make the right decision consistently, that's where he's falling down, not because of Wenger or the fact that he's been asked to do something impossible.
I don't think Fabregas would offer any less defensively than what Song offered today.
