Liverpool captain Jordan Henderson has undergone surgery on the groin injury he suffered in the Merseyside derby and will be out until April at the earliest.
Henderson was forced off in the first half of Liverpool's 2-0 defeat against rivals Everton at Anfield last Saturday and, after being assessed by the club's medical staff, it was decided an operation was necessary.
A Liverpool statement read: "Following further assessment with the club's medical team, Henderson has successfully had a corrective procedure carried out on the adductor injury. He will begin a rehabilitation programme immediately.
- Diogo Jota back in training; Virgil van Dijk steps up rehab
- Jamie Carragher on Liverpool problems and criticism
- Race for the top four: Paul Merson's verdict
"No specific timescale is being placed upon his return; however, the 30-year-old is ruled out initially until after the March international break.
"Henderson's recovery will be closely monitored by the club's medical department and his progression during his rehab work will determine when he is able to resume full training."
Henderson's injury means Liverpool will certainly be without their captain for the Premier League fixtures against Sheffield United, Chelsea, Fulham and Wolves.
The midfielder will also miss the Reds' Champions League last-16 second leg against RB Leipzig.
Klopp spoke about the absence of Henderson and the effect it could have on his team at Friday afternoon's press conference. He said: "They all have to be influential, with so many injured players at the moment. Most of them are leaders in this group even when they are doing their rehab.
"The best-case scenario? I don't know, it is not a quick one but we all hope that there is still a possibility for him to play a few games this season but we will see."
In addition, Gareth Southgate will be without one of his key players for England's internationals next month, with his side taking on San Marino, Albania and Poland in World Cup 2022 qualifying.
The news is yet another blow for Jurgen Klopp in a season in which injuries have hampered Liverpool's attempt to retain their Premier League title.
Virgil van Dijk and Joe Gomez both suffered serious knee injuries before Christmas and are uncertain to return this season, while fellow centre-back Joel Matip will definitely miss the rest of the campaign due to an ankle injury.
That forced Klopp to move Fabinho and Henderson from midfield into the heart of his defence. However, Fabinho has been injured for most of the past month, and Henderson is now also ruled out.
Klopp is now left with January signings Ozan Kabak and Ben Davies, plus Rhys Williams and Nathaniel Phillips, as his fit central defenders. The quartet have 11 Premier League appearances between them.
Carragher on Liverpool problems and criticism
Jamie Carragher has defended his criticism of Liverpool during the Premier League champions' astonishing slump and insisted injuries do not excuse their run of four consecutive home defeats.
Since January 1, Liverpool are 17th in the form table, with just seven points from nine games, including Saturday's 2-0 loss to Everton at Anfield.
Carragher told Sky Sports' Monday Night Football: "I said during commentary that l am sick of talking about Virgil van Dijk. And what l mean is that we have highlighted that throughout. Liverpool can't win the league. I said that after the last league derby game. They can't win the league with Van Dijk being out the whole season. That's the big picture.
"But the problems Liverpool have, with centre backs out and midfielders at centre back, does not mean they can't win home games against Everton, Brighton, Burnley, West Brom. Manchester United, maybe, and Manchester City are real top quality.
"It's not an excuse for individual games. No matter what problems you have as a Liverpool player, you can't not win at home for six games. That is not acceptable. It's not."
Pitch to Post Preview - Jamie Redknapp's Liverpool verdict. Plus: Chelsea v Man Utd build-up, and Tottenham's tricky Burnley test
On this week's Pitch to Post Preview Podcast, Jamie Redknapp joins Peter Smith to assess where Liverpool stand right now. The Sky Sports pundit explains why he thinks it would be a surprise if they make the top four and the reasons why he expects them to get back to their top level next season.
We also hear from Sky Sports football writer Jack Wilkinson, who assesses Chelsea's start under Thomas Tuchel, Man Utd's talk of the title, and gives his verdict on why the next couple of months could be key for the future of Jose Mourinho and Tottenham. Plus he shares his Pitch for the weekend!
Listen to the Sky Sports Pitch to Post Podcast on: Spotify | Apple | Castbox