Chelsea fans have taken to Twitter to celebrate their second signing of the summer, defender Kalidou Koulibaly. The Blues have announced his acquisition from Serie A side Napoli on a four-year deal. The Senegalese international has joined the Blues following the arrival of Raheem Sterling from Manchester City.As per The Daily Mail, the 31-year-old has cost the West London club between £32 million (€38 million) and £33.7 million (€40 million). Chelsea fans are seemingly in the mood to celebrate following the club's announcement of the deal.Let's take a look at some tweets from the Blues fans.Ben Jacobs@JacobsBenAgents Mendy and Jorginho. #CFC46849Agents Mendy and Jorginho. #CFC https://t.co/IdRCrX4B3pRJ@RJ_Goodthings@ChelseaFC What a signing! Technically proficient, physically strong, experience leader and warrior. Fantastic acquisition for our squad and club 9@ChelseaFC What a signing! Technically proficient, physically strong, experience leader and warrior. Fantastic acquisition for our squad and club 👊🔵💙ChelsTransfer@ChelsTransfer@ChelseaFC Best defence in Europe loading after next week.2063@ChelseaFC Best defence in Europe loading after next week.Ashwath Shetty@Chel_Ashwath71@ChelseaFC Kalidou is a Blue! I don't believe it, But Oh Yes! It's True!🤩From Naples he flew and now he's a Chelsea Blue Through and Through!! Welcome to Chelsea, Kalidou Koulibaly! 774@ChelseaFC Kalidou is a Blue! 💙I don't believe it, But Oh Yes! It's True!🤩From Naples he flew and now he's a Chelsea Blue Through and Through!! 💙Welcome to Chelsea, Kalidou Koulibaly! 💯🌟💙 https://t.co/InVcdjtFJmJAC @jorgeandcossio@ChelseaFC Jajajaja Koulibali > Maguiare and Varane2@ChelseaFC Jajajaja Koulibali > Maguiare and Varane💍Leeandro💍@Lee9ndro@ChelseaFC Score if you can5@ChelseaFC Score if you can https://t.co/HoVB1NO7wQAnane Yaw Cfc@YawCfc@ChelseaFC Welcome to the best club in the world 11@ChelseaFC Welcome to the best club in the world 💙💙💙💙💙💙💙💙💙💙💙💙 https://t.co/dl6tMKgLreKoulibaly has established himself as one of the best central defenders in Europe since his move to Napoli from Belgian side Genk in 2014. With his enormous ability as well as experience, he would be a huge asset to Thomas Tuichel's side.During his time at the Naples club, Koulibaly was 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 Senegalese defender has attracted interest from a host of top clubs across Europe. However, his dream move away from the Diego Armando Maradona Stadium never materialized with Napoli holding firm to keep him for so long.The new Chelsea defender is also quite experienced at the international level, having earned 62 caps for Senegal. He also led his country to the AFCON title earlier this year.Chelsea could be far from done in bolstering their backlineIt's hardly a surprise to see Chelsea bolstering their defensive ranks during the summer transfer window.Thomas Tuchel's side were left short of both options and depth at the back following the departures of Antonio Rudiger and Andreas Christensen.The centre-back duo left the Blues this summer for Real Madrid and Barcelona, respectively, both on free transfers.ESPN UK@ESPNUKKoulibaly joins up with Chelsea in Las Vegas 28718Koulibaly joins up with Chelsea in Las Vegas 🔵🎰 https://t.co/rTAPkWyiLpThiago Silva, Trevoh Chalobah and Malang Sarr were the only senior options for Tuchel at the heart of the defense before signing Koulibaly.Meanwhile, renowned journalist Fabrizio Romano has reported that the London giants are also pursuing deals for Nathan Ake and Presnel Kimpembe. If the Blues manage to sign both Ake and Kimpembe even after getting Koulibaly, Tuchel will have as many as six options at the heart of the defense.