Transfer expert Fabrizio Romano has confirmed Chelsea's second signing of the summer following the imminent arrival of Raheem Sterling.Romano has confirmed that the Blues have agreed a €40 million deal with Serie A giants Napoli for the transfer of Kalidou Koulibaly. The Italian journalist has reported that the Senegalese international will sign a long-term deal with a net salary of around €10 million. The 31-year-old will be the second signing at Stamford Bridge this summer after Raheem Sterling. The English international's move is yet to be officially announced, but as per Sky Sports, the Blues have a £47.5 million deal in place with Manchester City.Chelsea lost two of their star centre-backs this summer, with both Antonio Rudiger joining Real Madrid and Andreas Christensen joining Barcelona on free transfers.Fabrizio Romano@FabrizioRomanoChelsea are prepared to submit the final, official bid for Kalidou Koulibaly: €38m plus add-ons, up to €40m package. Napoli could accept, after opening proposal worth €35m turned down. #CFC Koulibaly agreed personal terms - while Chelsea are also working on Nathan Aké.363223873Chelsea are prepared to submit the final, official bid for Kalidou Koulibaly: €38m plus add-ons, up to €40m package. Napoli could accept, after opening proposal worth €35m turned down. 🔵🇸🇳 #CFC Koulibaly agreed personal terms - while Chelsea are also working on Nathan Aké. https://t.co/3LSFcL5u27Hence, it is quite understandable why Thomas Tuchel is looking for defensive reinforcements. The West London club currently have only Thiago Silva, Trevoh Chalobah and Malang Sarr as options at the heart of the defence.With his immense experience and quality, Koulibaly would certainly elevate the standards in Tuchel's backline. The Senegalese international has established himself as one of the best central defenders in Europe since his move to Napoli from Belgian side Genk in 2014.He has been named in the Serie A 'Team of the Year' on four occasions and was also named the 'Best Defender' in Italy in 2019.Over the years, the commanding defender has been consistently linked with big clubs across Europe but Napoli have held firm to keep him for so long. At the age of 31, he is set to finally have his dream move and could prove to be a solid addition to Chelsea.Koulibaly is also an established player at the international level. He has earned 62 caps for Senegal so far and led his side to the AFCON title earlier this year.Chelsea still have plenty of work to doChelsea have had quite an underwhelming start to their first transfer window of the Todd Boehly era and need to pick themselves up soon. The Blues will still need reinforcements in both defence as well as in attack, even after the signings of Sterling and Koulibaly.Fabrizio Romano@FabrizioRomanoKalidou Koulibaly already accepted Chelsea contract proposal yesterday night, just matter of time then it will be official. Boehly wanted to be fast as Bayern are leading the race for de Ligt. #CFCJune 28, official contact between his agent and Chelsea. Now, done. twitter.com/fabrizioromano…Fabrizio Romano@FabrizioRomanoKalidou Koulibaly, one more name in the list for Chelsea as they keep searching for two centre backs. He’s out of contract with Napoli in 2023. #CFCBeen told there are conversations also on Koulibaly alongside de Ligt, Aké, Koundé. Focus on Raphinha, and then centre backs.137351658Kalidou Koulibaly, one more name in the list for Chelsea as they keep searching for two centre backs. He’s out of contract with Napoli in 2023. 🔵 #CFCBeen told there are conversations also on Koulibaly alongside de Ligt, Aké, Koundé. Focus on Raphinha, and then centre backs.Kalidou Koulibaly already accepted Chelsea contract proposal yesterday night, just matter of time then it will be official. Boehly wanted to be fast as Bayern are leading the race for de Ligt. 🚨🔵 #CFCJune 28, official contact between his agent and Chelsea. Now, done. ⤵️ twitter.com/fabrizioromano…The London outfit's struggles in front of goal last season were evident. Bringing a striker should be their priority as the club have allowed Romelu Lukaku to rejoin Inter Milan on loan.It has been reported that Thomas Tuchel is eyeing as many as five signings this summer as he looks to close the gap with Manchester City and Liverpool.