The former Inter man scored a delightful winner after Marek Hamsik had been forced off due to injury in a dreadful evening for Rafa Benitez's men
The veteran forward waltzed his way through the home defence before firing a precise shot into the bottom corner on 81 minutes to seal maximum points for the visitors at the San Paolo.
Chances were few and far between in a fiercely-contested opening period, with Gonzalo Higuain providing the first real test for Antonio Mirante as he hit a point-blank effort straight at the keeper's legs.
Jonathan Biabiany and Nicola Sansone came close for the visitors before Higuain was again denied by Mirante after good work from Pablo Armero.
Marco Parolo had Pepe Reina at full stretch shortly before the break, before the midfielder steered Mattia Cassani's cross wide just after the interval.
Lorenzo Insigne was denied by the alert Mirante and the game looked to be heading for a draw before Cassano intervened, running at the defence unchallenged before finishing in style past Reina.
Rafa Benitez's poor evening was compounded by injury to substitute Marek Hamsik, who could now be a doubt for Tuesday's crucial Champions League clash with Borussia Dortmund.
Napoli remain three points behind Juventus in second having played a game more, while Parma leapfrog AC Milan into 10th after the Rossoneri's draw with Genoa.