Goals from Jose Callejon and Dries Mertens at Stadio San Paolo ended the champions' 14-game unbeaten run in the league.
With Roma winning 2-0 at Sassuolo earlier in the day, the Eternal City club are now nine points behind Juve with a game in hand and Italy goalkeeper Buffon warned that his side can ill afford to be complacent as they attempt to win a third-consecutive Scudetto.
He told the club's official website: "The championship is wide open. Roma have the chance to reduce the gap to just eight points and could go for a big final push."
However, Buffon believes the defeat to Rafael Benitez's charges could act as a wake-up call for the Turin club.
He added: "Sometimes a defeat can ultimately spark a great run of results, just like has happened in the past; hopefully it will have a similar effect in the future.
"Without looking to make excuses, I think Napoli deserved to win this game. We didn't express our style of play as best we can because of absences and the heavy fixture schedule."
