Amir Rrahmani set the tone of the match for Napoli on Sunday with an own goal after seven minutes.Â
Giacomo Raspadori equalised for Napoli 10 minutes later, but 19-year-old Spanish winger Assane Diao won it for Como after being set up by Nico Paz late in the second half.
Antonio Conte's Napoli had a chance to reclaim the Serie A lead from defending champions Inter Milan, who moved ahead by beating Genoa on Saturday, but will instead be in second for a match-up with Inter next weekend.
Napoli had drawn their three previous matches.
Como, coached by Cesc Fabregas, moved up to 13th place.
Meanwhile, third-placed Atalanta bounced back from a Champions League exit with a 5-0 rout at Empoli that was helped by a brace from Ademola Lookman.
Mateo Retegui and Davide Zappacosta also scored for Atalanta, who took the lead with an own goal from Emmanuel Gyasi.
Atalanta moved within three points of the top of the league.
Fourth-placed Juventus won 1-0 at Cagliari with an early goal from Dusan Vlahovic after they, too, were eliminated from the Champions League.
Hellas Verona beat Fiorentina 1-0 with a goal five minutes into second-half injury time from Antoine Bernede.
Fiorentina striker and former Everton player Moise Kean was diagnosed with a head injury after collapsing on the field in the second half.
Italy international Kean had been involved in an accidental collision with Pawel Dawidowicz and Diego Coppola, taking a knee to the face that left him with a cut above his eye.
Although Kean returned to the pitch after treatment with his head bandaged, the 24-year-old soon fell, initially appearing unresponsive, before being taken off on a stretcher in the 67th minute.
Fiorentina later confirmed Kean had suffered a head trauma and was in hospital for tests.
Kean made 32 Premier League appearances for Everton, scoring two goals, in the 2019-20 season.
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