We dont need a CB as much as we need a LB, CM and RW.
LB: Tierney or Sandro. Both attacking minded players. Sandro is better, but Tierney is younger and assumably cheaper. But if Sandro is the only way we can get rid of Darmian then Sandro would obviously be prefered.
CM: Box to box midfielder with some pace. Fred, Wylan Cyprien, Jean M. Seri would all do this job well enough.
RW: Dybala without a doubt. But it would be no point in buying him if Jose Mourinho is the manager. Then it would be better to buy and additional RB and field 2 RBs at the same time. Less likely to concede that way.
Either CB or RB: Koulibaly or Alderweireld for CB. Koulibaly is younger and arguable the better defender, but more expensive. For RB: Weiser would be the prefered one, but he signed for Leverkusen last week. Mounier would do the job.