Still fairly young. Still room to develop. Not uncommon for centre backs to mature a bit late.
Getting to the age where he should be showing clear signs of improving though. Time might not be running out yet, but it's getting to that stage. And where we are as a club, given that centre back is now the obvious place in need of upgrade, how long will he be given if he doesn't start improving soon?
There have been a lot of centre backs that have had "all the tools", which usually seems like a shorthand for quick and strong coming from many, that haven't stepped up. I struggle to see what he has that makes it likely that he will step up. But the right partner and Mourinho running the show will give him the best of chances.