How Chelsea will line up with Koulibaly
The Blues have been in search for defensive reinforcements following the free transfer exits of both Antonio Rudiger and Andreas Christensen earlier this summer, to Real Madrid and Barcelona respectively.
As a result, Todd Boehly has set about scouring Europe for potential targets, and has secured the services of Koulibaly with others such as Nathan Ake, Matthijs de Ligt and Jules Kounde among those on Chelsea's radar.
Chelsea acted quickly to complete the deal with Napoli for Koulibaly, and have now made the signing official.
It was previously reported that Chelsea were willing to match Napoli's asking price for the defender, and the Blues have now confirmed the move after weeks of negotiations between the two clubs.
Koulibaly is unlikely to be the only centre-back acquisition this summer, with Thomas Tuchel hoping to sign at least one more before the window shuts.
Manchester City's Ake looks the most likely right now, with the Dutchman keen on a return to Chelsea, while Tuchel is understood to be open to a move. But, how will Chelsea line up once Koulibaly arrives?
Chelsea with Koulibaly
There are several different ways Tuchel may set up with Koulibaly, but it suggests he may be open to playing with a back four. Of course, that's not known but the Senegal international could partner Thiago Silva at the back.
With Reece James and Ben Chilwell either side, that's a very impressive backline.
Mateo Kovacic and N'Golo Kante may be the first choice double pivot, while Mason Mount would likely play further forward. Raheem Sterling, Christian Pulisic and Kai Havertz may be the chosen front three.
Of course, Tuchel may simply opt to keep the 3-4-3 system that has served him well at Chelsea. If that's the case, another centre-back will come into the equation, with Pulisic dropping out.