Smalling and Jones need to replaced/relegated, as they are not good enough and worse still spend a lot of time out injured. How good a player is becomes moot when he spends most of his time on the physio table.
Rojo should be a backup CB imo. I dont see him improving much beyond what he has shown this season. But he is very good and suited to be that 3rd choice CB so that the quality of the defence doesn't drop much when either of the starting choices are absent due to suspension, injury, or NT (bailly ACN every 2 yrs)
If we exclude Smalling and Jones, we would need one world class CB that will be the defensive leader and organize the defence and another that is willing to be 4th choice but good enough to fill in. Keeping in mind that other top clubs like Madrid, ManCity, Chelsea etc are likely to be in the market as well.
Personally, I am fine with Lindelof+Keane. The former has shown enough talent at his age to expect him to develop to be a world class CB in a few seasons. His release clause is said to be about £40m.
Gimenez is another possibility I like, but with Godin declining with age, I just dont see Pathetico selling their best CB prospect unless he forces a move and/or we meet his release clause (reported to be £56m). He can also be a bit rash and think Lindelof will complement Bailly better.
Godin is too old and slow, while Bonucci has no reason to leave Juve, and if he does, I think Chelsea/Conte would be his preferred destination. Van Dijk is overrated imo.
For those that say Keane is not good enough (to be the 4th choice), who are the 2 CBs that are available that they would sign over him? As for the price tag, I think we would get a lower price than other clubs as the kid seems to prefer coming back to united. Also, we probably have a few players that would interest Burnley and I think it would be more beneficial long term to Burnley to not try to gouge us over a player we sold to them cheap. Of course, if another team is willing to cough up the cash, and the sell on clause is valid, then we might be better off taking the cash option.