Jurgen Klopp was frustrated by Liverpool's poor finishing after his side began their Champions League title defence with a 2-0 defeat away to Napoli at the Stadio San Paolo on Tuesday night.

Dries Mertens scored from the penalty spot with eight minutes remaining after Andy Robertson tripped Jose Callejon inside the box and Napoli substitute Fernando Llorente added a second in stoppage time to put the result beyond doubt.

Klopp’s side became the first competition winners to lose the opening game of their Champions League defence since AC Milan were defeated by Ajax in 1994 and the German was concerned by his players' wastefulness.

"It should hurt because there were opportunities for us," he told BT Sport.

"It was an open game with a lot of counter-attacks, but we didn't finish them off and that is a problem. In the second half it was a wild game, they were running and we were running."

Klopp was also unhappy about the penalty awarded to the home side after Robertson was adjudged to have fouled Callejon.

"I don't think it's a penalty," he said.

"What can I say, for me, it is clear and obvious no penalty. He jumps before any contact, we can't change that."

Virgil van Dijk and his team-mates were left to rue earlier missed chances.

He told RTÉ Sport: "We had opportunities today, particularly on the break to score but it is what it is.

"We had a lot of opportunities and we put them under pressure, especially on the break.

"We could have hurt them a little bit more.

"But this is always a tough place to come."

Liverpool’s talismanic centre-half added: "In various parts of the game we played some great football and we were very dangerous.

"The only thing missing was the goal."