The Brazilian’s thigh injury was initially believed to be minor, leaving him sidelined for Wednesday’s 3-0 win at home to City in the Premier League.
But speaking to reporters on Friday afternoon, ahead of Liverpool’s trip to take on Crystal Palace, Klopp revealed that Lucas is now facing a lengthy spell on the sidelines.
“Lucas is really unfortunate, a little bit more serious [than thought],” he said.
“I’ve never heard about an injury you can get like this!
“I don’t know exactly, but five or six weeks [out]. Not too good.”
The 29-year-old could be set to make his return in the Reds’ away encounter with Stoke City on the 10th of April.
Meanwhile, two other players to miss out on Wednesday night’s victory, Mamadou Sakho and Daniel Sturridge, could be in line to return against Palace.
While Sakho has a chance of featuring after suffering a head injury in Sunday’s loss at Wembley Stadium, Sturridge is set to return on Sunday having returned to full training.
“Mama today trained for the first time, a little bit, but the only [real] session we have is tomorrow – we have to see how he is then,” Klopp continued.
“[Sturridge] trained completely normally yesterday.”
After making five changes to his side for the win at home to City, Klopp indicated that he may reshuffle his pack for the trip to Selhurst Park.
“We made a few changes, and I think we made the right decision, and now we have to make a few [more] changes,” the German explained.
“Everything can happen, we will see who will come tomorrow, see the last training, make a squad and get ready.”