His current team. N A P O L I. And quality does play into it. Having a top quality defender next to Bailly and jones would fill them with confidence compared to Smalling. If not Koulibaly which leader is available? Alderweireld? Who doesnt look anywhere near as good as he did previously.
Wrong - Albiol is the defensive leader for Napoli or why do you think the 33yrs old is starting ahead of younger more athletic and individually better defenders. Last season, after 26 games, napoli had conceded only 8 goals in 20 games keeping 13 clean sheets with Albiol but without Albiol, they conceded 7 goals in 8 games with only 1 clean sheet. (FYI Alderweireld is a much better individual defender than Albiol).
Bailly, Jones and Smalling do not lack in confidence. Bailly just needs a calmer head to guide and reign him in, while Jones are Smalling are simply not that good. Lindelof lacked confidence due to the initial challenge of playing for a big club in a top league, but even he has gained more confidence in recent weeks. Adding Koulibaly will not change the confidence level of these defenders nor would an increase in confidence fix their shortcomings. At best Koulibaly would help to clean up their mistakes, similar to what DDG does.
Whether its Alderweireld or someone else, a defensive leader will likely improve our overall defense more than an individual contributor like Koulibaly would. After all, it is not like our defenders are losing a lot more 1v1 battles than average, but that our defence is not coordinated e.g. when shaw on left side plays Silva on-side at the far side, or when Lindelof and Herrera leave their man to close down on the same ball. Koulibaly is not going to carry shaw to the correct position, in fact his expectation that shaw would do the right thing might make it worse.
The need for a defensive leader surpasses the need for a great individual defender, cos that great defender cannot play all 4 positions in the backline simultaneously and would instead be overworked cleaning up after them. A defensive leader on the other hand would make the group function as a unit, help guide and correct mistakes on the pitch, and, reduce instances of blunders like uneven line or confusion over assignment. Even off the pitch a defensive leader remains quite valuable in developing younger defenders.
I have not see Koulibaly exhibit the needed traits and we should not gamble 100m hoping he would develop them when he gets here. If we buy Koulibaly, it should be purely for his individual contribution, which I dont think is worth 100m, particularly if you have a defensively sound coach like Mourinho.