"I was trying to cover as much of the goal," the Spaniard told Liverpool's website after making his Champions League bow.

"It was clear he was going to strike with the first touch. I jumped, watching the ball and tried to touch it with any part of my body. In the end I touched it with my right hand."

Adrian, signed last month on a free transfer from West Ham, is proving a useful deputy in the absence of the injured Alisson Becker, sidelined since the opening day of the Premier League season with a calf complaint.

However, Adrian's goal was finally breached in the 82nd minute when Mertens converted from the spot after Andy Robertson was adjudged to have upended Jose Callejon in the box.

Adrian guessed right and even got a hand to the penalty but the Belgian's effort was too powerful to keep out, which proved the decisive moment in the Group E fixture.

Former Tottenham striker Fernando Llorente sealed victory for Napoli in stoppage time as Liverpool's defence of their European crown started with a second defeat at this ground inside a year.

Adrian said: "It's my job as a goalkeeper, to try to save all the balls as much as I can and to try to help the team.

"I gave my best, I tried hard. But at the end of the day we lost 2-0, so I can't be happy.

"We knew it was going to be a hard game for us, like last season was. At the end of the game it was small details; the penalty switched the game for them totally and then we lost a bit of shape and conceded the second one."

The defeat ended Liverpool's perfect start to the season, having won all five of their Premier League fixtures so far after edging out Frank Lampard's Chelsea in the Community Shield at Wembley last month.

Next up is a trip to Stamford Bridge, and Jurgen Klopp's side will head to London looking for a 15th consecutive top-flight triumph.

Adrian added: "We are on a good run and we have to switch our minds now. (We have to) learn from the mistakes and then turn the page."