The Switzerland international has voiced his frustration with his side's first-half display at the Emirates Stadium and stressed they cannot afford to slip up against Marseille
The Serie A side were high on confidence after their win over Borussia Dortmund on matchday one, yet they failed to build on their good start as first-half goals from Mesut Ozil and Olivier Giroud proved to be too much for Rafa Benitez's men.
"We had a very poor first half. It is normal that when you play away from home in the Champions League, you will come up against a really tough team, but we really didn't expect to play that badly," Behrami told Uefa.com.
"We all knew that when you give away two goals to Arsenal at their home ground, it makes things even harder and is likely to make the result as bad as it can be."
Napoli travel to Olympique de Marseille on October 22 before hosting the French side two weeks later and Behrami has stressed that the next two matches will be crucial to their hopes of progressing.
"Next we have two decisive matches against Olympique de Marseille and they will crucial to us qualifying. We have had one good match and one bad match, so let's not get carried away," added the Switzerland international.
"Let's stay calm and get back to playing in the league on Sunday [against Livorno]."