You can't find better than him, for two reasons.
One, as
@milo pointed out, better than him would cost 50m. Secondly, a fitter player than him will demand more playing time, and look to move if he isn't getting it. After all, he's fit and he costs a lot, why shouldn't he play?
The fact that he's fit maybe 50% of the time is what keeps him from looking to play more. Which makes it convenient for us - better 50% availability than 0% because the bloke that replaces him wants out after one season on the bench, souring the mood in the dressing room in the process and probably tinkling off Poch.