Jose Callejon opened the scoring midway through the first half at the Stadio San Paolo before Dries Mertens' effort with nine minutes left to play secured all three points for the third-placed Partenopei.
The reigning Scudetto holders remain 11 points ahead of Roma in Italy, although they have played a game more than Rudi Garcia's men.
"Without going in search of whatever excuse, I have to say that Napoli deservedly won and that we we did not express ourselves adequately," Buffon told Mediaset Premium.
"The Scudetto is wide open because Roma are in great shape. They have all of the credentials to make it a race for the finish line should be slip up again.
"The loss does not bother me ahead of the Europa League last eight. In the end, sometimes losses precede big wins, which has often happened for us in the past. Let's hope that's the case again."
Juve visit Olympique Lyonnais on Thursday in the opening leg of their last-eight European tussle with the French giants.