Jurgen Klopp says he is unsure of the severity of Fabinho's injury, but says the outcome is "massive" after the midfielder limped off in Liverpool's 1-1 draw with Napoli.
Fabinho, who has played in all but one of Liverpool's Premier League games this season, limped off in the first half after team-mate Dejan Lovren attempted to tackle Hirving Lozano.
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He was replaced by Georginio Wijnaldum in the 19th minute and is banned for Liverpool's Premier League fixture against Brighton on Saturday, but Klopp will be keen to have him back for the Merseyside derby next Wednesday.
"The biggest thing is the Fabinho injury, that's massive," said Klopp.
"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."
Asked by reporters how serious the injury could be, Klopp added: "I don't want to say what I expect because I hope it is not serious.
"He has pain, which is not good. He couldn't continue. I hope it is not that serious. We'll know more maybe tomorrow or the day after."
Draw with Napoli adds to fixture stress
Klopp admits it is never easy at Liverpool and says he had not planned ahead to go to Red Bull Salzburg with a weakened side after their Champions League progress was halted.
The 1-1 draw with Napoli means Liverpool need a result in Salzburg in a fortnight's time, and means Klopp cannot rest some of his star players amid a hectic period of fixtures.
But Klopp said he never expected to have it easy enough to have the luxury of going to Salzburg not needing anything.
"If we have ambitions in the Champions League, we have to show that in Salzburg - and until then, we have to show our ambitions in the Premier League. There are a couple of games to play until then and we have to be focused on them - and we will be, no doubt."