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Firmino was brought on to replace Atletico, but his time there was only half an hour.
Roberto Firmino, Liverpool's striker, could miss more than four weeks due to a hamstring injury.
Fimino, 30, has scored four goals this season in Premier League games. He was injured in Wednesday's Champions League win over Atletico Madrid.
Jurgen Klopp, manager, said that losing the Brazilian was "a loss".
"We are still talking weeks. There are more than four. "I'd consider that to be a serious injury," the German said.
Curtis Jones, a midfielder, and Joe Gomez, defenders, have been ruled out for Sunday's visit to West Ham.
Klopp stated, "Curtis [to Firmino] is a totally different story." He was extremely unlucky during training, he had a finger that scratched his eye. We are still waiting to find out how long he will be back.
"He will return after the international breaks, for certain, but we must wait to see when he can get back to doing the proper stuff again.
"And Joey. Hopefully after the international break also."
Liverpool currently sit second in the Premier League, three places behind leaders Chelsea. They will not play again after Sunday's game, and they will host Arsenal on the league's 20th November.
