Giacomo Raspadori made Napoli provisional leaders of Serie A with the late winner in Saturday's 1-0 home win over Spezia.
Italy forward Raspadori struck the decisive goal in the final minute of a stodgy encounter at the Stadio Maradona, putting Luciano Spalletti's side one point ahead of Atalanta.
However they will lose their league lead should Atalanta beat promoted Cremonese on Sunday afternoon.
Raspadori was one of three signings made in quick succession by Napoli just after the season started, and the 22-year-old scored his first goal for the club on his second start after arriving on loan from Sassuolo.
Napoli will make his signing permanent next summer assuming he meets unannounced sporting objectives.
Spalletti's side are on a high after a strong start to the domestic season and thrashing Liverpool in the Champions League opener midweek.
They travel to Rangers on Tuesday before their first true title test against AC Milan at the San Siro next weekend.
Milan finish Saturday's action when they take on Sampdoria in Genoa after Inter Milan host Torino.
Spezia sit 14th on five points after their third defeat from the first six games of the campaign.