Well, either we get a RWB then or an out-and-out CB.
If we sell Zouma, we'll have only 3 out-and-out CBs in Rudiger, Christensen and Silva and I'm not sure Silva's fitness, especially, will hold up for an entire season. If one of them gets injured, then James or Azpi will be moved to the middle three, then leaving the other one for the RWB slot with no backup. Of course, Tuchel could switch to a back 4 or CHO can be used as a RWB but god knows where he is now in Tuchel's mind.