"The start was intense, that was clear", Klopp told reporters after the match.
The home side were rewarded for their early pressure in the 23rd minute, when Marin's corner was headed in by Milan Pavkov.
There was to be no way back for Liverpool as the Red Star fans celebrated a famous victory.
Reds defender Andrew Robertson has revealed that Liverpool is missing some sort of spark which helped them reach last season's Champions League final.
"They were on their bikes and we had to deal with the situation". The atmosphere singing-wise, noise-wise, was not a problem but the atmosphere after the second goal especially - in the whole club - they could smell it.
"It was a game we lost and that happened not 20 times so far".
"It wasn't like us and we need to move on and improve if we're going to go through to the knockouts".
A few we had to make and a few we wanted to but it is easy to say it didn't work out in the end.
Two first-half goals from €300,000 (£260,000) signing Milan Pavkov, the 24-year old scoring his first European goals in only his 11th appearance, were enough to beat the off-colour Reds.
Dejan Lovren and Naby Keita have made the travelling party and the former is in line to start while the latter could get a few minutes off the bench in the second half.
Klopp deserves vast credit for Liverpool's rise from mediocrity, but on the night in Belgrade, he was largely responsible for the defeat with his questionable tactics and team sheet.
Reacting after the encounter, Klopp was unhappy his side could not hold their own against their hosts and insists they must step up the ante for their game which is trip to Paris to battle Ligue 1 giants PSG later this month. The 29-year-old Englishman does not offer the dynamism that the Brazilian offers and this was evident in Liverpool's struggles, with their attack lacked cohesion, and The Reds failing to get a shot on target until the 71st minute.
"We had a bad game in Belgrade and we never ignore that".
"We gave them too many set pieces before they scored, so they could kind of train it".
Red Star Belgrade have won both of their European home matches against Liverpool, also triumphing 2-1 in October 1973 in the first leg of a Last 16 tie in the European Cup.
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