Milan, who had taken seven points from their previous three games, have dropped to 13th in the table with 14 points.
Milan: AC Milan are back in crisis mode after their home defeat by Fiorentina and the last thing they want at the moment is a trip to Napoli's daunting San Paolo fortress.
However, that is exactly what awaits coach Massimiliano Allegri and his struggling team on Saturday before they travel to Belgium for a potentially decisive Champions League tie against Anderlecht on Wednesday.
Napoli have dropped only two points in their six home games in Serie A and won 4-2 at Genoa last week. Milan, meanwhile, were beaten 3-1 by Fiorentina, a result which means they have lost half their 12 league games so far.
Former striker Zlatan Ibrahimovic reminded Milan of what they were missing with four goals, including his stunning bicycle kick, for Sweden against England on Wednesday.
Ibrahimovic was among the players offloaded as Milan attempted to slash their wage bill and balance the books in the close season.
The Swede's striking partner Antonio Cassano and defensive lynchpin Thiago Silva also departed and the Brazilian, now with Ibrahimovic at Paris St Germain, said he was shocked by Milan's poor start.
"It's difficult to judge from a distance but I have never seen a Milan like this," Silva told Gazzetta dello Sport on Thursday. "Six defeats in 12 matches just isn't Milan. I hope they can pick themselves up."
Chief executive Adriano Galliani issued his customary support for the coach after the Fiorentina defeat although alarm bells began ringing when he waited until the following day rather than doing so immediately after the match, as is more usual.
"It is obvious that we are not happy," said Galliani. "We evaluated everything and decided that there's no guarantee that changing the coach would improve the situation.
"We have confirmed Allegri, he has done well in the past two years and this is the decision we have made."
Milan, who had taken seven points from their previous three games, have dropped to 13th in the table with 14 points, leaving them much closer to bottom club Siena (7) than leaders Juventus (31).
Napoli are third with 26 points in yet another impressive season.
"We have nine more points compared to this time last season and I am glad to see my team never gives up or stops looking for the victory to the very end," said coach Walter Mazzarri.
"I train an exceptional group of players. There is a real sense of union and we all care about each other."
Leaders Juventus are at home on Saturday to fifth-placed Lazio who are fresh from their 3-2 win derby win over AS Roma. Crisis clubs Sampdoria and Genoa meet in their derby on Sunday when at least one of them will end a run of defeats.
Sampdoria are 17th with 10 points after slumping to seven consecutive league defeats, prompting a protest by fans outside the team's training ground on Wednesday. Genoa are one point behind in 18th after losing their last five games.