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Juve's 2-1 defeat at SPAL a day earlier left them 20 points clear at the top of the table with six games left to play, and if Napoli had failed to beat Chievo an eighth consecutive title for Massimiliano Allegri's men would have been confirmed.

Koulibaly headed the visitor in front after 15 minutes at the Marcantonio Bentegodi and Carlo Ancelotti's men were dominant until Milik scored from 20 yards in the last half hour of a game Chievo never looked like taking anything from.

The result was made more emphatic by Koulibaly's second before Bostjan Cesar's late consolation, leaving Napoli with a mathematical chance of catching Juve but, barring a spectacular collapse from the Turin side, second place looks likely for Ancelotti's men.

Koulibaly, who was the victim of alleged racist abuse during Napoli's midweek Europa League defeat at Arsenal, stooped to head Dries Mertens' corner into the net at the near post to put his side ahead.

Vlad Chiriches somehow completely missed the target when presented with a free header at close range eight minutes later but Chievo failed to muster the kind of response that would have forced their visitors to be more clinical in front of goal.

The game was put beyond the host when Milik collected the ball 20 yards from goal and had time and space to pick his spot in the bottom left corner of the net, which he bulged with a low, bouncing shot.

Jose Callejon launched a rising drive towards goal on the half-volley 20 minutes from time only to see it sting the palms of Stefano Sorrentino in the Chievo goal, but there was no stopping Koulibaly's piledriver 10 minutes from the end as he lashed a low, left-footed shot into the net from six yards out.

Chievo captain Cesar scored with a looping header from a corner but that turned out to be the last action of the game as Chievo were well beaten and condemned to Serie B next season.