It was marginal. They were conceding at a similar rate, just had a much improved overall team iirc - although I might be wrong. Van Dijk is the case I can recall where one signing completely changed the course of a defense. One or two cases is arbitary and does not prove anything. It might be due to overall improve in coaching that coincided with the signing of a particular player.
In our team, the problem stems far beyond lacking a leader or something along those lines. There's obvious lack of quality among our starters, proper coaching, and the style does not help matters too. One defender would change little.
the situation is rare makes the examples rarer.
Cannavaro's impact was not minimal. Madrid had an aged backline, with Salgado and Carlos as full backs and Helguera (a converted DM to CB) and the like of pavo, mejier and bravo as CB. They had recently bought Ramos and Marcelo who were still green.
The previous season the defence was all over the place, but relied on experience and strong attack to hide their defensive vulnerabilities.
when capello was hired, his prime defensive signing was a 32yr old cannavaro, who brought organization and leadership to that backline and improved the players like ramos. it was less about his individual defensive contribution, cos he was already in decline. Anyone that watched the games could see what he brought. (Mourinho did something similar in bringing in Carvalho when he was at madeid).
Looking at our current defence we lack individual quality and also lack the leadership and organization of a cohesive unit. these are not things rectified by coaching (cos he can't get on the field and stop shaw from playing Silva onside on the other flank). while having a stronger attack or midfield might help hide the problem, it wont make it go away and would always rear its ugly head when the defence is tested. A bad defence is a bad defence.Same way, a solid defence lets you be more open when attacking and gamble more.
currently, we basically have two groups: the young with potential (bailly, lindelof, shaw and dalot) and the old and mediocre ( smalling, rojo, jones, young and valencia). To make our defense better, we either buy quality proven defenders (like varane, koulibaly etc) which would cost a lot of money or buy cheaper younger talents. The latter option would last longer and has less risk of flopping but needs a system to groom them.
thus we need to get an experienced defensive leader who can help organize the defence and also help with the development of lindelof and co. That is the best way to transform our defence without spending half a billion.
For Koulibaly, i dont think he fits the role and buying him would only gain us his individual contribution. Also the rumored price tag and age are of concern. i would prefer a cheaper option for the role.