The Stadio San Paolo outfit were kept to a 3-3 draw at home in their Serie A clash with Udinese, coughing up 2-0 and 3-2 leads to only take a point from the fixture.
Benitez's men require both a win over Arsenal in Naples, and hope Marseille halt Borussia Dortmund in France, to qualify for the Champions League knockout stages on Wednesday.
But the former Liverpool gaffer said they will not be doing so, should they commit errors such as they did on Saturday.
"We made two mistakes from set plays, including an own goal, and we are suffering because we make mistakes we should not be making," Benitez told Sky Sport Italia.
"This result means we still have to improve, as I've said many times. We should be in control of a match and then we make a mistake. We are the ones who make the errors and we have to do better.
"Of course we all expect Napoli to be at the level they were at early in the season and for the side to have more points by now. We cannot be satisfied with this and I am the first to be unhappy."
Benitez's men require more points than Dortmund in the final matchday of the group stages to reach the Champions League Round of 16, but he said defensively they will need to be sound against an in-form Arsenal side.
"Clearly it's a different competition and if we score goals the way we did today, then we have to do better when defending.
"Those two errors on the set plays really should not be happening and they changed everything.
"The way to beat Arsenal is to keep a tight team, have intensity, move the ball around and close the spaces."
Udinese boss Francesco Guidolin was delighted with the result, as his side - who entered the clash with 12 goals in 14 - put three past Benitez's Napoli, who he also wished well in the Champions League.
"I am satisfied for my players, as in my view in terms of mentality and approach we have been doing very well over the last month," Guidolin told Sky Sport Italia.
"It was a shame we conceded the 3-2 (deficit) when we had just equalised but it's also true the team kept its cool to get the 3-3 and a point we fully deserved.
"Our performance against Inter was the worst in my time here but before and after that we took the right approach, intensity and realised each point is precious this season.
"We are back where we belong, which is to fight for safety and not look ahead to Europe. We accepted that and the results are starting to come.
"I believe Napoli's attack, when working at speed, can cause problems for anyone and I hope they can qualify for the next round of the Champions League by beating Arsenal."
