The Brazil international had a scan on Thursday that confirmed an injury to the anterior cruciate ligament in his left knee.
A statement on Liverpool’s website said Lucas “will undergo surgery and unfortunately be ruled out for the remainder of the current campaign”.
Lucas added: “I am really sad with the news because I was enjoying (it) a lot on the pitch.
“I am sure I will come back stronger and I will achieve everything that I dream with the LFC shirt.”
The 24-year-old Lucas, injured in a clash with Chelsea’s Juan Mata, has been one of Liverpool’s best players for the past two seasons.
And he has been in fine form during the past fortnight, starring in Liverpool’s Premier League and League Cup wins over Chelsea and Sunday’s 1-1 league draw with leaders Manchester City.
In particular, he has formed a successful midfield partnership with Scotland international Charlie Adam, a pre-season signing from Blackpool.
However, Liverpool do have other options in midfield with Jay Spearing able to fill a similar defensive role, Jordan Henderson waiting in the wings, and Jonjo Shelvey recalled from a loan spell with second-tier side Blackpool.
Ideally, Liverpool would like to have captain Steven Gerrard back to full fitness as soon as possible.
The England midfielder has made just five appearances this season, having had his start to the season delayed following a groin operation in April and Gerrard has not played since October 22 because of an ankle infection.
Liverpool, sixth in the Premier League table and 12 points behind leaders Manchester City, are away to Fulham on Monday.