The 2006 World Cup winner says his former clubs look to be in a two-horse race for the Scudetto, but his heart is with the Partenopei, who are captained by his brother.
The retired World Cup winner began his career with hometown club Napoli, before taking in two spells with Juventus, during which time he was awarded the FIFA World Player of the Year award - now the Ballon d'Or - in 2006.
"The match will not be decisive, but it will give [us] important information," the 39-year-old told La Gazzetta dello Sport. "The preparation of [Juventus coach Antonio] Conte and [Napoli coach Walter] Mazzarri is the secret of the two teams [success].
"I do not make predictions. It does not seem right to do so. It is not a matter of the heart versus the head. I have great respect for Juve, whom gave me so much, where I won two championships. The heart, however, dominates this profession. I am Neapolitan and Naples are the team from where I grew up."
The former defender also spoke of his family ties to Napoli, which is now captained by younger brother Paolo Cannavaro.
"Paul [Paolo] was and is one of the protagonists of the great revival of the club after the failure," said the 136-capped international, who also talked about the two team's title chances. "Juve are the most complete team in the league and Napoli are the only club that can currently counter them. Character could be decisive, the will to win and above all, the belief in one's own strength. In short, neither of them will be afraid of the other."
Juventus and Napoli share the summit of the Serie A table going into Saturday's tie, with the former currently ahead on goal difference after seven games played each.