Koulibaly is one of the top three defenders in the world, according to Antonio Conte, who wanted to sign him during his time at Chelsea.
Conte was keen to take Napoli centre-back Koulibaly to Stamford Bridge during his two seasons at the helm in west London, with a reported €58million offer turned down in 2016.
The 29-year-old has remained a key part of the set-up at Stadio Diego Armando Maradona and will hope to lead Napoli to a positive result against Serie A leaders Inter on Sunday.
Since making his Serie A debut in 2014-15, Koulibaly ranks first among defenders in the competition for clean sheets (76), tackles won (315), recoveries (1,526), successful passes (12,852) and successful passes in the opposition half (5,291).
He will need to call on all that experience when he goes up against Inter striker Romelu Lukaku, whose haul of 21 league goals this season is only bettered by Cristiano Ronaldo.
Asked about Koulibaly, Conte told a news conference: “He is one of the best two or three defenders in the world and I wanted to bring him to Chelsea.
“He’s actually improved since then. Lukaku has improved a great deal too. It’ll be an interesting duel, but both sides will have to work as teams to create goalscoring opportunities.”
Inter are on track for a first Scudetto since 2010, heading into this weekend 11 points clear of second-placed Milan with eight games remaining.
The Nerazzurri have been criticised for a defensive, counter-attacking style despite their healthy advantage, but Conte believes he is the target of the negativity rather than his team.
“I realised that it’s all about me. That’s the basic problem. I don’t mind that, as long as they don’t touch Inter,” said Conte.
“There are still many points up for grabs and we’ll go to Napoli for the victory. We cannot start calculating or holding back because my experience tells me it’s counter-productive.
“We mustn’t fall into the trap of taking the Scudetto for granted because it hasn’t been decided yet. We know the sacrifices we’ve made to reach this position, but must be humble to realise there are still some steps to go before we can make that dream come true.”