Napoli, involved in the Europa League round of 16, rested a number of players and also had goalkeeper Alex Meret suspended for the clash with Sassuolo.
Sassuolo went ahead seven minutes after halftime when Domenico Berardi turned in Jeremie Boga's pass from close range.
Napoli appeared to be heading for a fifth league defeat of the season until Insigne pounced on a mistake by Sassuolo captain Francesco Magnanelli and curled the ball in with four minutes left.
Unbeaten Juventus, who have won 24 of their 27 games and drawn three, have 75 points with Napoli on 57.
