Benitez enjoyed a promising first season at the helm, winning the Coppa Italia crown and securing UEFA Champions League football with a third-placed finish.
However, Napoli finished 24 points behind champions Juventus and seven adrift of Rudi Garcia's Roma.
The Spaniard expects those two clubs to lead the way again in the forthcoming campaign, even though Juve have lost the services of coach Antonio Conte.
"For the Scudetto, Roma and Juventus start with an advantage," Benitez told Sky Sport Italia.
"We also saw Inter have constructed a younger team with more intensity.
"All three of them will be difficult to beat, but we mustn't forget Fiorentina and Milan too."
Napoli earned a 2-0 pre-season win over PAOK on Saturday with goals from Jose Callejon and Josip Radosevic.
Benitez was heartened by the display and believes new signing Kalidou Koulibaly, who arrived from Genk in May, will strengthen his side's resolve.
"If we make good signings it is because [director] Riccardo Bigon and his staff work so well," he added.
"Kalidou Koulibaly is a player with the characteristics we were looking for in defence."
