The Slovakian midfielder admitted that the Gunners deserved to win after the Italian side conceded two early goals
Goals by Mesut Ozil and Olivier Giroud in the first 15 minutes handed the Gunners at 2-0 victory in the Champions League Group F clash, and the Slovakian was left to rue his side's poor start to the match.
"At the start, they were devastating and we played very badly," Hamsik told reporters after the game.
"You cannot play like that in the first 15 minutes of a match. It gave them a lot of morale, and there was nothing we could do to turn the situation around."
The Naples outfit had gone into the game buoyed by victory over Borussia Dortmund on matchday one and an encouraging unbeaten start to the Serie A season, though they were without star striker Gonzalo Higuain for the clash in north London.
Hamsik, though, was quick to talk down the Argentine's absence as a reason for the defeat.
"He is very important, but he is not the only one. We can grow as a team, and we must grow," added the 26-year-old.
"We just gifted them those first 15 minutes. It's unforgivable, you just cannot do that."
Napoli remain second in Group F following the defeat, with a trip to Olympique de Marseille next up on October 22.