I would personally. I would let him see out his contract and try to persuade him to stay whilst doing so.
He's our best defender by a country mile and, because of his contract and injury record, we'd get a very low transfer fee for him. Much less than what we paid. There's no way we could attract a defender of half his quality to the club, certainly not for the amount we would get for him. He's worth a lot more to us if he stays, even if it's only until the end of his current contract.
In addition to that, he's a character and a leader and you shouldn't underestimate how much that brings to the group. We don't have many of them. I'd actually make him captain next season.