Carlão and Marco Benassi came into the starting eleven to replace the suspended Emiliano Moretti and Afriyie Acquah but made the worst possible start.
After only seven minutes, José Callejón fired the away side into the lead as he slid the ball past Joe Hart from an acute angle.
Toro were struggling to create clear cut chances, but in the early stages of the second half, Lucas Boyé almost scored his first Serie A goal but curled his shot narrowly wide.
The Granata were made to pay for that miss, as Lorenzo Insigne doubled Napoli's lead following Dries Mertens' assist. The Belgian then scored a goal of his own as he beat Joe Hart at his near post with a fierce shot.
Quick fire goals from José Callejón and Piotr Zielinski followed as Torino were dealt a heavy defeat in their penultimate home game of the season.