Napoli could finish the weekend trailing Serie A leaders Juventus by 11 points after Carlo Ancelotti's wasteful side stuttered to a 0-0 draw at Fiorentina, as Lautaro Martinez scored the only goal to get Inter Milan back winning against promoted Parma.
Both sides start their Europa League campaigns this week after crashing out of the Champions League with Napoli travelling to FC Zurich and Inter Milan away to Rapid Vienna in their round of the 32, first leg ties.
Fiorentina goalkeeper Alban Lafont produced a string of saves to deny Piotr Zielinski and Dries Mertens as Napoli failed to score away from home for the third game running.
"We dropped two points that we would've deserved for attacking from start to finish," said Ancelotti.
"In some ways, it was an impeccable performance, but we weren't clinical enough in front of goal and were forced to make do with a point."
"We must now keep a low profile to rediscover stability. We showed positive things tonight, but we need to produce further performances like this to gain consistency."