I've not seen Koulibaly play for some time now but from what I've read, he's not as good as he used to be. So I wouldn't call him world class, but maybe I'm wrong.
We don't really need a world class CB IMO because Our FBs are very good defensively and if we can add a good DM like Partey or Zakaria, we should be more than fine defensively. But if Upamecano or Konate become available for reasonable price, I wouldn't be against us buying a CB.
I really hope we buy up all these great young players like Upamecano, Konate, Zakaria, Sancho, Grealish, Bellingham. I know we can't buy them all but if we can get one of Upamecano or Konate plus Sancho and Zakaria, we will have a very talented squad. Grealish or/& Bellingham would be the icing on the cake. And if we invest in those players, we won't have to invest much for a few years