The Serie A giants took the lead through Eljif Elmas' 11th second strike but lost 3-2 after the final whistle with Mario Rui’s red card before the half-hour mark having huge effect on the game.
Not being pleased with the result, Senegalese defender, Koulibaly admitted ‘We were already thinking about Sunday’s game with Fiorentina' adding that his side ‘have much to learn’ after the shocking defeat.
"It was a tense finale, because their assistant manager was winding us up during the break," Koulibaly told Sky Sport Italia.
"I don’t want to look for any excuses, we could’ve done much better. We lost this match and have much to learn. It takes a lot of work to win every single fixture.
"There’s disappointment and regret, but we need to take it one game at a time. I think we were already thinking about Sunday’s game with Fiorentina and that was a mistake. If we play with our minds elsewhere, we’re going to struggle.
"This match taught me a great deal, on a personal level too. The coach was very disappointed, he told us before the game how we should approach it. From tomorrow, we must find the right focus for a difficult test against Fiorentina."
The Partenopei are enjoying a good start to the season under Italian trainer Luciano Spalletti as they have won all their six Serie A games and are the league leaders at the moment.
Elsewhwere in Europe, Napoli are still without a win after two games, sharing spoils in their first Europa League encounter with Leicester City.
