"We are in a difficult period and we didn't manage to play our usual football," he told UEFA's official website.
"During every season you have to face a spell like this, but we want to return to our usual standard as quickly as possible.
"It was not easy tonight after hearing the score from the other game, but we have to look at ourselves in the mirror and try to understand how to start playing our best football again.
"We are a solid group and I’m sure we will do it.”
Marek Hamsik echoed his team-mate's comments and said it was a tough defeat to take, with Napoli having gone ahead after just two minutes courtesy of Piotr Zielinski's goal before an equaliser from Nicolai Jorgensen.
Feyenoord were then reduced to 10 men late on when Tonny Vilhena was sent off but still managed to sneak the win thanks to Jeremiah St. Juste's stoppage-time header.
"We were not watching what was happening in the other game in Ukraine, we were just focused on our game," he said.
"It’s painful to lose with a late goal but we had to do better earlier in the game, we had to play with more pace in midfield.
"The defeat at Shakhtar made everything complicated for us from start in this group. We are still eager to do well in Europe and we will try to do it in the UEFA Europa League instead."