After successfully get Koulibaly seems Chelsea hasn't stopped to buy defender players and rumour is they want to buy Josko Gvardiol from RB Leipzig...
Of course they can stop at that, for now they have only two recognized, senior team defenders in persons of T. Silva and Koulibaly, that isn't enough, Chelsea is a team that competes for all titles, so as they progress in domestic cup competitions and the CL, they are going to be playing far more games than the average teams, so they need as many players as possible, there will be fatigue and injury problems, so Tuchel wants to make sure that he uses this summer transfer window to counter those 'future' problems.
Having said that, I have not heard of any rumor about this particular player, that doesn't mean it isn't true, they have been linked with a couple of central defenders in the past two weeks, soon we will know whom they will sign, i think Chelsea still has to sign two central defenders to be complete at the back,
remember T. Silva isn't the youngest player in the world right now, so he will have more issues with injuries and fatigue than is normal.