His pace is an issue, but he is also dropping far too deep. We're not trying to defend en masse like Burnley do, and it allows crosses to be deeper and leaves space on edge of the box. It's a pretty basic problem, but he turns slow, that if he does push out, he'll leave space in behind.
He's good on the ball and he's a proper old school defender like Jags in terms of eyes open, face first, I'm stopping the ball. But I could see a situation similar to Mertesacker at Arsenal, where he is useful with height at both ends and a decent defender, but it limits the ability of the team to push as far forward as most imposing teams do. Would probably be better as part of 3 at the back.