The Brazil forward lasted just over half an hour as a substitute during the 2-0 win at Anfield before he was forced off and is not expected back immediately after the upcoming international break.
“Losing Bobby Firmino is a blow,” Klopp told reporters. “We’ve known for two or three days that it’s a serious injury so that means we talk about weeks still. I don’t know exactly but more than four, which I would consider as a serious injury.”
The Liverpool manager said the 30-year-old was a “quick healer”, but that he was unlikely to be available for selection at home to Arsenal in the Premier League on November 20.
Firmino has scored six goals in 11 appearances for Liverpool this season, with the Merseysiders currently on a 25-match unbeaten run in all competitions, equalling their club record.
Defender Joe Gomez, who has a calf problem, and midfielder Curtis Jones, suffering from an eye injury, missed the Atletico victory and will also be unavailable for Sunday’s trip to to high-flying West Ham.
“Curtis is a completely different story. He was very unlucky in training — he got a finger that scratched the eye,” Klopp said in a club statement.
“I had a lot of injuries to players in my career, that was not involved. He is on the way back but we have to see how long exactly,” the German added.
“He will be back after the international break, for sure, but in the international break we have to see when he can start doing proper stuff again.
“And Joey (Gomez), hopefully after the international break as well.”