The Partenopei were 2-1 up and heading for a win, when Rolando Bianchi scored deep into stoppage time to earn his side a share of the spoils.
After the game, Benitez said that he was angry with the decision to award the goal, as he felt that there was a push on Raul Albiol in the build-up.
"Albiol told me that he was pushed on the corner at the end and I think that there was a foul on him there," he told reporters.
"They harassed us well and made things difficult for us but in the second half we were faster and better in attack but we had to find that third goal and have to show more superiority.
"We must also work much harder to improve our prowess from dead ball."
The result leaves Napoli third in the Serie A table, 12 points adrift of league leaders Juventus.