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.
The veteran keeper admitted Juve could have no complaints about the defeat, but felt their hectic fixture list and injury problems caught up with them.
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."
