I think he's done well this year, just a bit of a knock.
I was in the same boat as you until recently, but there really is no getting around the fact he's brilliant. Rather than sell him and risk whatever shite we may bring in, let's keep him and try to add to him. Spurs have done the same thing with King. You dont go selling players like that, you just love it when they're fit for 20 league games a season in total, whatever games they may be.
I think we'd regret selling him, every week we'd see him in another league and doing brilliantly, and we'd kick ourselves for selling him. Not just ability, but attitude and leadership, we'd miss it big time.
Same as with Gary Cahill, I think having him, Agger and Skrtel (and Coates) to rotate would have been fantastic, it would have sorted out our centre back position for the next 5 years or more.