Okay, let's look at it this way: VVD - best defender in the world, but sometimes plays like he's bored with how good he his. Needs an occasional kick up the arse. JoGo: Ridiculously quick and mobile, but seems to have lost his mojo since the City mauling last season. Get's injured at least once a season. Fab: Fab, but we haven't seen him up against a Deeney or Giroud yet. And he's still the best DM in the league, imo. Matip: Reliable when he plays - and you can rely on him not playing most games.
You need four CB's to get through a full Prem and CL season, and someone to deputise in the early rounds of domestic cups. I'm all for giving Williams a run out CB in the League Cup as he's tall and aggressive in the air. But we need a fresh CB for a full season: one from the lower leagues (who are really struggling for money atm) might be an idea, as might a European loan. Don't want that Diego Carlos from sevilla though - he looks expensive and rash, imo. And don't need to throw our empty wad at Koullibaly either - tremendous as he is, £70m on a fourth CB would be a ridiculous waste of limited resources. A happy medium is what's required.
But all that said - if we can strike a good deal for Sarr as well atm, I would get him. The front three are pushing 30. They'll need replacing in the next few years, even with Mbappe and Sancho coming next year.