The only CB we need is one who will lead our back line no point of bringing in sidegrades or slight upgrades.
Slight upgrades, I think there are a lot of centre backs out there who would be huge upgrades on the defenders we currently have at centre back personally, though it is hard to get a good fit, look how many centre backs City had to sign and Wenger didnt manage to sign a real top quality one after Sol Campbell.
Presently I wouldnt trust any single one of our defenders to lead a title challenge:
Jones cant stay fit and has actually regressed since we bought him, he was a better defender at Blackburn
Rojo has done ok at centre back, but just ok and as been lucky not to get a few red cards
Bailly looked a rough diamond in his first season, he has looked just roug this season, still think he has a chance though
Smalling has probably been our best performer at centre back but he is awful on the ball and doesnt seem to have intelligence, seems totally reliant on his physical attributes
Young has done ok at full back but isnt a first choice starter especially with his age next year
Valencia same as Young
Shaw has the ability, does he have the attitude?
Dalot, looks very promising, time will tell
Darmian, well he is off isnt he
We have Laird and possibly Tuanzabe to add as squad players next season to replace a couple of those players, but yes we definately need upgrades in my opinion, at least one