There is a lot at stake for Red Star Belgrade heading into Tuesday night's game, as they will be eliminated from Europe's elite club competition if they lose to Liverpool FC and Napoli win at home against Paris Saint-Germain.
"The start was intense, that was clear", Klopp said.
Mohamed Salah fired over the bar shortly afterwards to sum up a poor first half for Liverpool, with Daniel Sturridge also missing an early chance for the visitors when he volleyed over the bar from close range.
They got a hard group stage draw when PSG and Napoli came out of the hat with them but they continued their excellent European form with an opening round win against the Ligue 1 champions to put themselves in pole position. "They deserved it with the passion they showed".
A loss to French champions PSG would leave them facing a nail-biting shoot-out with Napoli and Liverpool midfielder Lallana knows his team have no excuses for their predicament.
The Reds had chances to get themselves back into the contest but failed to take them. Four points from their last two games would be enough to guarantee the Reds' progress to the last 16. Before that, Liverpool had scored in 14 straight games. They're probably the only ones this season that we haven't, for whatever reason.
Pavkov, a surprise starter up front, gave Red Star a 22nd-minute lead when he headed home from the edge of the six-yard box after a Marko Marin corner from the right.
Despite Liverpool's improving form Firmino has struggled for goals, scoring just once in his last 10 matches. They are now huge games for the rest of Liverpool's season.
Klopp regularly conducts analysis with his players the day after a game, having reviewed the match himself at least twice but Wednesday's debrief assumed a more critical air given the nature of defeat and the precariousness of Liverpool's position in the group. They scored a goal and then completely out of the blue they scored a second and the atmosphere completely changed.
Almost three decades on, Red Star don't have a household name in their squad and needed to qualify for the group stage through the playoffs.
"We have to make sure it doesn't happen again".