Luiz, Ivanovic, Kalas, Cahill, Terry. That's 5 for two spots. Normally you'd need 4, but with 3 of those filling in at other positions and Terry's age and knees, we needed someone else. If Kalas will only play cup games he should go out on loan. Since he's staying, he needs to be utilised. Anyone else shpuld only come if Cahill or Terry (or God forbid Luiz or Bran) are set to leave. We also have Christensen and other youths for cup games, Essien for emergencies, and Omerou and others can be recalled if needed. Any transfer in needs to be balanced by one out, and our two semi-disposable CBs are HG which makes them more valuable to retain.
If van Ginkel is our creative mid, the only other dealings I'd look at are: improving the DM (supplemented by someone leaving), Shaw ir some other young LB, a HG backup GK so Blackman can go get regular minutes elsewhere, maybe a striker (supplemented by either Torres/Ba leaving or Lukaku leaving on loan), a potentially worldclass CB to replace Terry/Cahill. Honestly, if Mou has faith in the youths, I'm fine with the squad. I'd rather transition this year and make Lukaku, KDB, Kalas, van Ginkel, Bertrand and whomever into Chelsea-level players than spend big on the Cavanis and stunt their development.
Also this Bruma kid is apparently very good and his situation means he'll be cheap. I'd sign him.