Liverpool travelled to Portugal on Tuesday night in search of a place in the semi-final of Europe's elite competition.
Klopp's side boast a 2-0 aggregate lead following last week's routine victory at Anfield.
Lovren impressed that night, but the Croatia centre-back missed training at Melwood yesterday and the German manager says that he is too ill.
“Dejan is ill. He had a bad night, the Doc was there and he had no chance to travel. That is the reason for that," he told the Reds' official website.
“If you feel not well and had a bad night, we all know a day later and a high intense game, it doesn’t sound like exactly the thing you should do."
Klopp has a number of people able to replace Lovren - who earns £100,000 per week on Merseyside [Spotrac] - this evening.
But Fabinho, normally a central midfielder, is probably the best option, given that Joe Gomez has barely kicked a ball for Liverpool since December, while Joel Matip can be hit and miss.