Liverpool coach Jurgen Klopp has conceded an injury to Fabinho is "massive" following the club's 1-1 draw with Napoli in the Champions League on Wednesday.
The 26-year-old lasted just 20 minutes at Anfield as he was forced from the field with an apparent ankle injury following a heavy challenge in midfield.
Fabinho was already set to miss the Reds' weekend match against Brighton in the Premier League through suspension but could now miss further games as Klopp looks to keep momentum during a congested month.
The German admitted the early signs are not great for his midfielder, whose long-term loss could prove a big blow for Liverpool.
"The biggest thing is the Fabinho injury, that's massive," Klopp told BT Sport post-match.
"It's early and we all hope it's not too serious but it's pretty painful and in an area you don't want to have pain, around the ankle."
Fabinho has already made 18 appearances for the Reds this season with the club poised to now play nine games across a busy December.
After drawing with Napoli, Klopp's side face a crucial match against Red Bull Salzburg on December 10 where a loss could see them miss out on the Champions League knockout stages.
The Reds manager cut an animated figure on Wednesday as he saw his team struggle to break down their Italian opponents despite dominating possesion.
And the former Borussia Dortmund boss was ultimately handed a yellow card for engaging with the officials, though he stressed his comments were a plea for leniency rather than anything aggressive.
"'Don't do that', that's what I said, because they made a big fuss of the situation," he said.
"We all said there's an opportunity for both teams to have a quick restart and he had things to say. So I said 'Why do you do that, please don't do that'."
Liverpool are currently top of Group E, one point ahead of Napoli and three clear of Salzburg.
Salzburg eased past Genk 4-1 on Wednesday to keep their hopes of progression alive, with teenage sensation Erling Haaland continuing his breakout season in the competition with a goal and assist.