Koulibaly scored the only goal of the game on Sunday as Napoli stole a late win in Turin to close the gap on the six-time back-to-back reigning champions to just a single point.
The Senegal international has been linked with Chelsea for some time, but Arsenal, United and even Barcelona have reportedly emerged on the scene as well, with United in particular thought to be pursuing defenders in the transfer market at the end of the season.
Napoli, who also own Premier League target Jorginho, are no lightweights when it comes to negotiations and made Juventus trigger a €90m release clause to buy Gonzalo Higuain in 2016.
And Italian journalist Fabrizio Romano for CalcioMercato explains it is that Higuain saga which is responsible for Koulibaly's release clause being so high as well as having a certain caveat.
After losing Higuain, Napoli seemingly sensed possible danger and had successfully renegotiated Koulibaly's contract within a matter of weeks, raising the release clause to a largely prohibitive €70m.
Were he to go for that price, it would make him the second most expensive defender of all time, which would at least line Napoli's pockets in the search for a replacement.
It would also represent a huge profit for the club after buying French-born Koulibaly for just €8m from Belgian club Genk in the summer of 2014.
The interesting aspect of Koulibaly's contract is that the release clause apparently cannot be triggered by other Serie A clubs, something that was obviously not the case with Higuain's clause. It would appear that Napoli learned their lesson and made the terms more specific.
After watching Higuain make the switch to Juventus, Napoli were weakened and saw the team they were chasing grow stronger in one swift movement.
That won't happen again, although it means the Premier League clubs chasing Koulibaly will have to pay a premium. And there is no ceiling on what Juventus would have to pay...