NOt sure teh centre backs are the problem. Koscielny needs a bit more time, and hopefully Vermaelen and Djourou can form a more commanding and secure CB pairing.
The problem IMO is Song. Something has happened to him recently, he was incredible, but the last 5 or so matches, he hasn't got forward as much, and he's been awful on the ball.
Also, one quality attacking player is necessary.