Italian journalist Fabio Santini seems to be the one and only source for this, and according to him the transfer is supposed to happen next summer.
I don´t put much trust in this, though. Firstly, Koulibaly has not been very good this season. Secondly, there are other areas to be prioritised. Thirdly, Tuanzebe and Bailly. Sure Ole would like to see what they can do before signing another DC, as both have shown great potential when staying fit.
Unless, of course, Ole is planning for the departure of Lindeløf, pluss Bailly perhaps.