The Blues conceded 54 Premier League goals - the worst among the traditional top six teams in the division.
And their defensive problems reared its ugly head again when they were beaten 2-1 by Arsenal in the FA Cup final at Wembley.
Chelsea have already spent €93m on signing Timo Werner (€53m) and Hakim Ziyech (€40m) to reinforce their attack for the upcoming season, with Kai Havertz likely to follow suit.
Leboeuf believes his former side should prioritize signing a center-back by citing an example of how Van Dijk became the final piece of the jigsaw when he joined Liverpool.
“[He needs to improve] all of them. That’s the problem,” the Frenchman said about Frank Lampard’s challenges.
“The goalkeeper is a big issue, but you have to go to the main [issue]. If you’re looking at the main issue, it’s the central defenders.”
“[Kurt] Zouma is doing great, but I’m not confident about [Antonio] Rudiger, I’m not confident about [Andreas] Christensen. I don’t know what will happen with [Fikayo] Tomori, but I think they need a big guy.
“You saw what happened with Liverpool when they got Van Dijk, he changed everything. Even Lovren and Gomez looked better alongside him.
“You need a big guy to make sure he’ll lead the defense. Look at Ramos. [Raphael] Varane is a fantastic player, but is he still the fantastic player when he plays for France as when he plays for Madrid with Sergio Ramos? He’s better with Real Madrid.
Leboeuf played for Chelsea between 1996 and 2001, winning six trophies during his time at Stamford Bridge.
