Koulibaly struck his first league goals this season to put Napoli 17 points behind Juventus.
Juventus, who lost 2-1 at SPAL on Saturday, could have clinched the title if Carlo Ancelotti’s second-placed Napoli had not beaten Chievo.
Massimiliano Allegri’s side will have to wait at least until next weekend at home against Fiorentina to seal the Scudetto with a record-equalling five matches remaining.
“We have to continue like this because we have no room for error on Thursday,” said Koulibaly ahead of next week’s Europa League quarter-final return leg game against Arsenal. The Italian side trail 2-0.