The Brazil international damaged the anterior crucial ligament in his left knee during Tuesday's 2-0 victory over Chelsea in the Carling Cup quarter-final at Stamford Bridge.
Lucas was carried from the field on a stretcher after a clash with Juan Mata and his - and the club's - worst fears were confirmed by a scan. The 24-year-old was inundated with messages of goodwill on Twitter and he responded in kind, writing: "Just to confirm that I will be out until the rest of season."
He 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. I am sure I'll never walk alone."
The midfielder's absence is a huge blow for Liverpool as Lucas had been on of their best performers not only this season but the previous campaign as well.
In the last fortnight, in particular, the Brazilian has shown his worth with impressive performances against Chelsea in both the Barclays Premier League and Carling Cup victories and also in the 1-1 draw at home to Manchester City on Sunday.
But the ideal scenario for manager Kenny Dalglish would be a swift return for captain Steven Gerrard, who has made just five appearances this season, having had a delayed start after recovering from a groin operation in April, and has not played since October 22 because of an ankle infection.
The news about Lucas was confirmed in a short statement by the club, which read: "Liverpool FC said a scan had confirmed an injury to the anterior cruciate ligament in Lucas Leiva's left knee. The Brazil international will undergo surgery and unfortunately be ruled out for the remainder of the current campaign."