The Spaniard replaced Walter Mazzarri at Stadio San Paolo in May and has only seen fit to start club captain Cannavaro twice all season.
The 32-year-old defender has not featured since his dismissal in October's Serie A defeat at Roma - in which he picked up two yellow cards in the space of 25 minutes after being brought on as a substitute.
But Materazzi believes Benitez has stepped out of line and Cannavaro is good enough to hold his own in Napoli's title-chasing side.
"Discrediting Paolo Cannavaro in such a short time didn't seem right because he's done a lot for Napoli," he told Radio CRC.
"Paolo is no less talented than any of the other players in that role. In my view, Benitez has won a lot, but he's too presumptuous."
Napoli face a tricky week which will see them host Arsenal on Wednesday with their UEFA Champions League hopes hanging by a thread, while Mazzarri returns to Naples with Inter in Serie A on Sunday.
Materazzi believes the Milan club will gain extra motivation from the 52-year-old's return, but expects a strong performance from Napoli.
"Napoli have bought so many players and spent a lot of money," he added. "The squad is higher quality than last season, so they have to do better than last season.
"The Azzurri have been strong in the league, but they've had too many setbacks. The problem is that when the key players aren't in shape, the whole team suffers.
"I expect them to play a good match against Inter. There's also the return of Mazzarri to Naples, and the coach wants to win."
