Napoli paid the price for playing a weakened side as they dropped UEFA Champions League points in 0-0 draw away at Racing Genk on Wednesday.
After beating holders Liverpool in their opening Group E game, coach Carlo Ancelotti left out Lorenzo Insigne and dropped Dries Mertens to the bench against the Belgian champions
Genk had conceded six goals in their opener against Salzburg.
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Arkadiusz Milik made only his second start of the season in attack and his rustiness showed as he struck the woodwork twice.