Is he going to stop us making defensive errors in critical positions? Yes.
Is he going to be a leader to help organise all those around him? Yes.
Is he going to bring years of top level experience and keep a cool head in big games? Yes.
Will he make stupid decisions and jump in like Otamendi leaving a one on one for forwards? No.
Will be pushed off the ball or stumble in possession like Stones? No.
Is he better at CB than a 35 year old converted DM? Yes.
Does he have more recovery pace than any of our current CBs thus reducing the likelihood of a key chance? Yes.
Will he improve our team? Yes.
Is he going to stop Salah or Mane one on one? Why the fuck wouldn't he? I saw Garcia tackle Mane the other day, why won't a world class defender be able to do the same? To say 'No' categorically is nonsense. EDIT - as others have pointed out, he's already done this for Napoli!
You're asking the wrong questions. All those questions basically undermine someone like Kompany. Are you seriously telling me that VK's presence last year in the run in was basically null and void because we weren't having problems in other positions last year? Clearly bollocks. Koulibaly doesn't solve every problem throughout the team (OBVIOUSLY ffs) but he does solve the ones we're having that our specific to CBs. He is better than Stones, Otamendi, Fernandinho and Garcia currently. Probably possibly better than Laporte, if not level. He improves us, and that's all that will matter.