Lucas picked up the injury while taking his penalty in the shootout against City, and the manner in which he picked up the injury left his manager bemused.
"It's more serious than we thought. I've never heard of an injury like this, after taking a penalty," the German told a news conference on Friday.
Klopp called for consistency ahead of the trip to 14th-placed Palace, with Liverpool sitting six points outside the Champions League places in eighth place in the table.
"Six points is not the world. I would like to say from now on we would take every point but we have to show it," Klopp said.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"This is the most important part of the season. I don't think about the top four. We have a lot of points to get and we should start getting as many as we can."
Palace, who could be without in-form striker Connor Wickham for Liverpool's visit, beat the Merseyside club at Anfield in the reverse fixture in November, and Klopp is keen to return the favour.
"It's Crystal Palace," he said. "We lost to them at home and we need to take points back."
(Reporting by Simon Jennings in Bengaluru)
