While the home side have struggled at the start of their title defence, Milan were top of Serie A before losing last weekend; and both have since experienced different fortunes in Europe.
Match preview
Rudi Garcia's future as Napoli boss came into question when his team lost back-to-back matches just before the international break, but beating Hellas Verona and then Union Berlin in the space of a few days has relieved some pressure on the champions' head coach.
The ship was steadied last Saturday, when Khvicha Kvaratskhelia struck either side of the break at Stadio Bentegodi in a 3-1 win, before the Partenopei travelled to Germany in midweek.
After requiring a late own goal to beat Braga, and then suffering a 3-2 home defeat to Real Madrid, Napoli retained second spot in Champions League Group C on Tuesday night, with Giacomo Raspadori scoring the only goal at Berlin's Olympiastadion to leave them on six points at the halfway stage.
Ahead of next month's reverse fixture in Naples, Garcia's side must tackle top-four rivals and then a fellow Campanian club, Salernitana, in Serie A, where they sat fifth after nine rounds; five points adrift of leaders Inter.
While speculation persists about their coach's position - and the long-term future of star striker Victor Osimhen - Napoli can only concentrate on closing that gap, and victory on Sunday would see them move within one point of Milan.
However, they have lost two of their last three home games in the league - as many as in the previous 23 - and expectation may weigh heavily on their shoulders at a packed Stadio Maradona.
In their last nine top-flight meetings with Milan, Napoli have both won three and lost three, but their most recent encounter brought elimination from the Champions League quarter-finals, shortly after a 4-0 league defeat in early April.
In addition to a 1-1 draw that saw them progress to the Champions League's last four, Milan have won each of their last three Serie A away matches against Napoli - representing their best such run in history.
This time, they travel south on the back of two chastening defeats, the first coming at home to likely Scudetto rivals Juventus, who left San Siro with a 1-0 win after the Rossoneri had been reduced to 10 men during the first half.
Although they put up a strong resistance following Malick Thiaw's red card, third-choice goalkeeper Antonio Mirante was eventually beaten by a deflected shot from ex-Milan midfielder Manuel Locatelli, who had scored the winning goal in the equivalent fixture seven years before to the day.
Defeat was Milan's second in nine league matches this term - the other a 5-1 thumping by old foes Inter - but they have won seven times already and sit just behind their city rivals in the standings.
After losing to his former club, coach Stefano Pioli sought an immediate response in midweek, when his side visited the French capital to take on Paris Saint-Germain, but following a humbling 3-0 loss at Parc des Princes they have now failed to score in their last five Champions League matches.
Rock-bottom of the 'Group of Death', also featuring Borussia Dortmund and Newcastle United, Milan must surely need three points from next month's reverse fixture in Italy's second city, but first they return to domestic duty on Sunday.
Napoli Serie A form:
D
D
W
W
L
W
Napoli form (all competitions):
W
W
L
L
W
W
AC Milan Serie A form:
L
W
W
W
W
L
AC Milan form (all competitions):
W
W
D
W
L
L
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Team News
As 2022-23 Capocannoniere Victor Osimhen has been ruled out for the next few weeks with a hamstring injury, Napoli must again improvise up front, where midweek matchwinner Giacomo Raspadori is favourite to beat Giovanni Simeone to selection.
Midfield regular Frank Anguissa is close to returning from a thigh problem, but Juan Jesus remains sidelined so Natan and Amir Rrahmani should start in central defence.
Elsewhere, Piotr Zielinski has scored four league goals against Milan in the past and is the only midfielder with double figures for assists since the start of last season in Serie A (10) - only forwards Rafael Leao (11) and Khvicha Kvaratskhelia (13) can boast more.
After suffering a downturn in form earlier this year, the latter has produced 14 shot-ending ball carries and seven chance-creating carries in domestic games this season (21 combined), and only Barcelona's Ousmane Dembele has a higher such tally (22) across Europe's top five leagues.
Meanwhile, Milan's attacking options have been boosted by the news that Noah Okafor has resumed full training after missing out against PSG, although Luka Jovic may not be passed fit in time to feature at Stadio Maradona.
Sunday's game also comes a little early for Ruben Loftus-Cheek, who has not played since the end of September, while Samu Chukwueze, Ismael Bennacer, back-up goalkeeper Marco Sportiello and suspended centre-back Malick Thiaw are also unavailable.
Having both featured on Wednesday, Mike Maignan and Theo Hernandez can return from the Serie A suspensions that ruled them out last weekend.
Napoli possible starting lineup:
Meret; Di Lorenzo, Rrahmani, Natan, Rui; Cajuste, Lobotka, Zielinski; Politano, Raspadori, Kvaratskhelia
AC Milan possible starting lineup:
Maignan; Calabria, Tomori, Kalulu, Hernandez; Musah, Adli, Reijnders; Pulisic, Giroud, Leao