Kaboul needs to be cut some slack.
How many players come back from lengthy injuries and hit top form straight away? Sandro is in equally poor form at the moment but seems to be immune of the same level of criticism. The only possible exception I can think of is King but he was simply a freak a nature. If (and going by the past couple of seasons it's a big if) Kaboul can stay injury free for a consistent period of time he will prove to be an asset. He's an excellent defender and was absolutely vital to us before.
One a seperate note, I think central defence is a position that you can really judge a player as a single thing. The good centre back partnership elevates both players to another level and it's so important to their effectiveness for the team. Consider that Kaboul has come back into a team devoid of confidence, from an extremely lengthy injury, with constantly changing personnel, it would be premature to just toss him away in the summer.