Laporte didn't need any time to adapt, Fern didn't need any time to adapt, Ederson didn't need any time to adapt.
Instead of just making irrelevant blanket statements if you look at the coaches Koulibaly has played under - Benitez, Sarri, Ancelotti - then he's got loads of experience under top coaches and with Sarri he's played under a coach who has as similar a style of play to Pep's as exists.
In fact if you wanted you couldn't find a coach who put more Pep-like demands on centrebacks, both in possession and out of possession than Sarri at Napoli.
So I doubt he'd need the same period of adaptation.
He's an incredible player, it just comes down to the cost Napoli demand as to whether it's worth it for a player you're only gonna get 4 or 5 years out of.