There is a popular saying in football. The attack wins you games but the defense wins you titles. Barcelona's fate in the recently concluded campaign aligns with this notion.Despite having Lionel Messi pulling the strings upfront, the Blaugrana's poor defense has repeatedly proved to be an obstacle. They went very close to winning the Liga title but a 2-1 defeat to Granada in April ended their title challenge.The Catalans were also knocked out of the Champions League in embarrassing fashion with a 5-1 loss to Paris Saint Germain on aggregate.Barcelona had the chance to go to the top of La Liga. They blew a 1-0 lead and lost 2-1 against Granada at home 💀 pic.twitter.com/GRwMt5yu0y— B/R Football (@brfootball) April 29, 2021Barcelona have been let down by their defense several times in the last few years, especially in crucial matches. If they want to compete for La Liga and the UEFA Champions League next season, they surely need to improve their options at the heart of defense.The Blaugrana have already signed Eric Garcia from Manchester City this summer. But the Spaniard is just 20 years old and still has a lot to learn to succeed at the top level. Barcelona will know they need to sign a high-profile centre-back to help the team going forward.To do this, they would need to cough up substantial amounts in transfer fees or negotiate by offering players in exchange.Potential centre-back options for Barcelona#3 Kalidou KoulibalyKoulibaly: The defender is said to have handed in a transfer requestNapoli centre-back Kalidou Koulibaly is a target for many big clubs in Europe. The 29-year-old has been linked with a move away from Napoli almost every summer in the last few years.Koulibaly is fantastic at keeping opposition attackers at bay. He is a shrewd defender who excels at interceptions and tackles per game and is really effective at nullifying counter attacks.According to Whoscored, he had an impressive tally of 72 tackles and 78 clearances for the Italians across Serie A and the Europa League last season.The Senegalese uses his immense physicality to his advantage and does really well in aerial duels. He has a good sense of positioning, passes the ball impressively and feels comfortable in possession. These are qualities that would help him fit in at Barcelona.It is believed Koulibaly has handed in a transfer request to Napoli. This could make things easier for Los Blaugrana to snap him up. As per Transfermarkt, the player has a market value of €48 million. Barcelona could bring the figure down by offering players in exchange.