Rosonneri scored the solitary goal courtesy of Ismael Bennacer’s strike from Milan’s counter-attack to give them the 1-0 lead going into the second leg of the Champions League quarter-final.
Napoli will also be without Andre-Frank Zambo Anguissa, who was harshly sent off.
Stefano Pioli’s sides had recorded a 4-0 in Naples in the Serie A just ten days ago.
Napoli made an aggressive start in the opening 90 seconds. Davide Calabria stopped the visitors from going ahead with a goal-line clearance when Rade Krunic gifted the ball to Kvicha Kvaratskhelia.
Mike Maignan was also called into action when he pulled long-range Andre-Frank Zambo Anguissa and Piotr Zielinski strikes over the bar.
Rafael Leao showed his mettle when he sprinted forward from his half and drilled inches wide of the far bottom corner.
Milan eventually broke the deadlock with a counter-attack. After turning between two defenders, Brahim Diaz sprinted forward and tried combining with Leao, but the over-hit return pass came to Ismael Bennacer, who fired in at the back post to score his first Champions League goal.
This was the first goal conceded by any Italian club in the knockout rounds this season.
The hosts almost doubled their lead in the first-half stoppages. Simon Kjaer’s header from a corner thumped the underside of the bar and bounced back out of the line.
Maignan also fingertipped Elmas’ looping header onto the crossbar when back-pedalling furiously.
Anguissa, who was initially booked, got the second yellow in under four minutes, both for challenges on Theo Hernandez. This is seen as harsh because he seemed, although, on the second, he seemed to get the ball.
Kim Min-Jae will be suspended for the return leg after being booked for a slight push on Alexis Saelemaekers.
The card is seen as dissent than the foul itself.
Napoli still threatened despite being a man down, and Maignan stopped Di Lorenzo’s strike at the near top corner.
Mathias Olivera also nodded a Di Lorenzo cross over from six yards.
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