Klopp was asked post match what went wrong for his side during the defeat, before firing back: "I only have 10 fingers."
A first-half double from Milan Pavkov was the difference for the home side with the Serbian striker's long-range effort in the 29th minute a standout.
The result changes the complexion of Group C with Napoli now in top spot but only two points between first and fourth with two games remaining.
Klopp rued his side's poor showing which was a long way from their 4-0 win against Red Star in the reverse fixture at Anfield two weeks earlier.
“We had the first big chance with Daniel (Sturridge), a very big one. Each goal in a game like this leads the game in a specific direction," Klopp said.
“The atmosphere, noise-wise, was not a problem for us, but after the second goal they could smell it.
“We made it too easy for them. They deserved it with the passion they showed.”
Klopp declared the result was a one off and insisted his team had nothing to worry about, despite Liverpool losing three straight Champions League away games for the first time in the club's history.
“We lost our mojo in the game, not in general,” he said.
“We have to make sure it doesn't happen again.”
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