The Premier League leaders fell behind in the 82nd minute after Jose Callejon went down easily under pressure from Andy Robertson, before Fernando Llorente’s stoppage-time strike secured Napoli a 2-0 win.
“I don’t think it’s a penalty,” Klopp told BT Sport.
“What can I say, for me, it is clear and obvious, no penalty. He jumps before any contact, we can’t change that.”
Liverpool became the first holders to lose their opening game of the following Champions League campaign since AC Milan in 1994, as their front three of Mohamed Salah, Sadio Mane and Roberto Firmino were all guilty of wasting chances.
“It should hurt because there were opportunities for us,” added Klopp. “It was an open game with a lot of counter-attacks, but we didn’t finish them off and that is a problem.
“We controlled moments but had not enough chances in the end. We made decisions that were not right and have to accept the result.
“It was very often the final ball that was not right.”
Liverpool next face a difficult trip to Stamford Bridge to take on Chelsea in the Premier League on Sunday, and host Salzburg — who thrashed Genk 6-2 on Tuesday — in their second Champions League Group E game on October 2.
