We are not going to be able to get a CB who is better or equal to what we already have for anything less than £35 million in the current climate.
Personally I'd keep Mangala. He's different and can physically handle the stronger attackers. If we're defending a 1 goal lead away at a long ball side like WBA or United then he'd be the perfect option to bring on.
Plus I don't see his lack of passing ability to be a major problem. The majority of teams that we play just turn up to defend and all you want your CB to do is give it to the likes of Yaya or KDB who normally drop deep or pass it to the more accomplished passer is Stones, Vinny or Ota.
Kompany is a shoe in to start when fit. The club won't give up on Stones who is also a top defender and Otamendi is a criminally underrated player who is easily one of the best CBs in the PL.
Trying to sell the position of 4th choice CB might be difficult to a player but it looks like Eli will be willing to take a squad role and will cost nothing!