It didn't take long for VAR to become a talking point as the video refereeing system made its debut in the Champions League group phase.
Liverpool coach Jürgen Klopp was left livid over a penalty awarded to Napoli that helped the Italian side beat the defending champion 2-0 on the tournament's opening night on Tuesday.
Dries Mertens converted the penalty eight minutes from the end of an entertaining match after a contentious foul by Andy Robertson on Jose Callejon.
Substitute Fernando Llorente then sealed it in stoppage time with his first goal for Napoli — a measure of revenge after playing on the Tottenham side that lost last season's final to Liverpool.
Klopp thought that Callejon exaggerated the contact after Robertson was adjudged to have tripped the Spaniard in the area.
“I’m pretty sure there are different views on that. When a player jumps before there is contact it is not a penalty,”Jurgen Klopp, Liverpool manager
Klopp then wondered aloud how the referees came to the decision.
"I'm not the best loser but I'm not a bad loser. It was a decision made by human beings. As long as human beings make the decisions there is potential for failure. The referees and the VAR were all German, and they have experience with the VAR in Germany," said Klopp, who is also German. "I'm sure they will find an explanation on why they did the right thing."
Still, Klopp was not altogether discouraged.
"That was the game changer tonight and we have to accept the result," he said.
“We are really critical with ourselves but it was not a really, really bad performance.”Jurgen Klopp, Liverpool manager
The latest chapter in what is becoming an intense rivalry between the two attack-minded clubs, which also met in last season's group phase, was full of action.
The atmosphere inside the newly renovated Stadio San Paolo was electric from the start as both sides attacked relentlessly from the opening whistle.
"It was very balanced match," Napoli coach Carlo Ancelotti said. "They're the best squad in Europe and they were again tonight, because they forced us to be extraordinary."
Liverpool became the first reigning champion to lose its opening game since AC Milan was beaten by Ajax in 1994.
Last season, Napoli also beat Liverpool 1-0 at home before the English side won by the same score in England — denying Napoli a place in the knockout rounds.
Napoli and Salzburg, which routed Genk 6-2, took the early lead in Group E.
"I hope it's not decisive," Klopp said. "Give us at least a few more games to change the table."
Napoli countered Liverpool's established attacking trio of Mohamed Salah, Roberto Firmino and Sadio Mané with a lineup featuring four forwards: Lorenzo Insigne and Jose Callejon on the wings surrounding Dries Mertens and Hirving Lozano.
Salah was on full sprint from kickoff but couldn't find a way past Napoli's centre back Kalidou Koulibaly.
Liverpool goalkeeper Adrián — in his Champions League debut — denied Fabian Ruiz twice in one action before Lozano headed the second rebound into the net only to be whistled offside.
As dazzling as the offensive display was, the defense was just as impressive.
Joël Matip lunged to head away a cross intended for Lozano that seemed a sure goal then Ruiz sprinted back at the other end to intercept what would have been a tap-in for Salah.
The frantic pace continued, with Firmino heading just wide of the far post from a corner in the 44th.
Possession in the first half was split exactly 50-50.
The fireworks continued immediately after the break when Adrian hung in the air to deny a volley from Mertens with one hand at the far post.
There was a rare defensive mistake when Kostas Manolas gifted the ball to Salah in a great position but Napoli goalkeeper Alex Meret extended to glove the Egypt international's shot wide.
It was only with 15 minutes to go that the pace slowed, with both teams visibly exhausted on a warm evening.
Then came the decisive penalty.
Adrián lunged to his left and got a hand on Mertens' spot kick but couldn't stop it.
In the final minutes, Napoli fans' whistled so loud every time Liverpool had possession it was deafening, and it seemed to have an effect when Virgil van Dijk gifted the ball to Llorente, who rolled it past Adrián with ease.
"We put in practically the perfect performance," Callejon said. "We've faced them so many times that we know them tactically by now and did everything right."
After the final whistle, Manolas led the celebrations by dancing in the center circle wearing Salah's No. 11 shirt.
Meanwhile, Ancelotti — who added another chapter in his long, personal history against Liverpool — consoled Klopp.
“I told him to relax, because if he loses here, he tends to go on and win the Champions League. I reassured him.”Carlo Ancelotti, Napoli coach
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