Liverpool manager Jurgen Klopp said that Roberto Firmino has suffered a "serious" hamstring injury and is unlikely to return after the international break next week.
Firmino limped off as a substitute during the 2-0 Champions League victory over Atletico Madrid at Anfield on Wednesday.
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The club released a statement confirming the injury news on Friday and Klopp added that Firmino might be unavailable until December.
"Losing Bobby Firmino is a blow," Klopp told a news conference."I don't think I need to say how big it is. It's a serious injury.
"We talk about weeks -- more than four I consider as a serious injury. Then we have to see. Bobby is a quick healer."
The 30-year-old was scheduled to join Brazil during the international break but has been replaced by Real Madrid's Vinicius Junior.
Firmino will miss Sunday's clash at West Ham United and is likely to be out of action for the Premier League matches against Arsenal and the Merseyside derby at Everton on Dec. 1.
The striker's absence adds to a growing list of injured Liverpool players with Curtis Jones, James Milner, Joe Gomez and Naby Keita all out of action.
Following a disappointing draw against Brighton last week, Liverpool are third in the table and five points behind leaders Chelsea.
However, Klopp added that Liverpool don't see themselves as underdogs in the title race.
"We don't consider ourselves in that way -- we want to be in it, but whatever role we have I don't care," he said. "With our quality we should be in and around the top of the table.
"We don't have to talk about quality of [Manchester] City and Chelsea. The league is incredible. There's no game where you can go and say 'we can rest him and him and him'. In the last two or three years, it has been the best league in the world."