It's a problem position however you look at it; we've a lot of good/decent CBs but no top quality ones. Our best one is probably Jones and he's a sick note. Bailly and Rojo both have plenty of ability but are prone to mistakes, they're as hot and cold as they come. Smalling can defend but apart from that he's nowhere near where we need to be, can't play out from the back/organise the rest of the defence and he's our most senior CB.
We lack leaders and players capable of playing out from the back which limits the sort of football we can play, no one can organise the backline and it's clear as day that it needs upgrading. For the last couple of years we've had a low number of goals conceded, not due to the quality of defence but due to a combination of de Gea (let's not kid ourselves, without him we could easily have conceded another 10 or so goals) and playing defensively. City have least goals conceded, by the way.
A few teams around us have quietly (or not so quietly) been building very good defences; Liverpool have Van Dijk, Robertson and Clyne who are all very solid players, TAA also looks promising. City have Mendy, Kompany, Otamendi and Walker with some very good back ups in Stones and Laporte. Tottenham may lose Alderweireld but already have Vertonghen and Sanchez along with some solid full backs in Davies, Rose and Trippier. Chelsea have Rudiger, Christensen, Alonso, Azpilicueta and are linked with Koulibaly. On paper I'd say that our defence may be starting to fall behind.
We need to be careful by saying "well we've conceded least goals so CBs are fine" but in my opinion they're very clearly not fine, I think our defence needs reshaping big time but it's tricky doing it all at once. Years of neglect have left us with two ageing wingers as our first choice full backs and an endless string of rotating CB partnerships that change weekly due to injuries/form/mistakes whatever.