Liverpool could be without Roberto Firmino until close to Christmas due to a "serious" hamstring injury.
The Brazilian was a half-time substitute for Sadio Mané at half-time of Wednesday's 2-0 Champions League win over Atletico Madrid at Anfield.
However, Firmino's appearance lasted just 33-minutes, before he was replaced by Divock Origi after going to ground with a hamstring problem.
"Bobby is not good news with a serious hamstring injury," said Klopp, ahead of Sunday's Premier League trip to West Ham, "really unfortunate.
"We don’t know exactly how long he will be out but it will be not now after the international break directly. We have to work on that.
"Losing Bobby Firmino is a blow. We have known for two days it was serious.
"We are talking weeks, still. More than four. I'd consider that a serious injury. Then we have to see. Bobby is a quick healer."
Liverpool could be without Firmino for one of their busiest periods of the season.
The West Ham game precedes the international break, but after it comes games with Arsenal, Porto and Southampton before November is out.
A hectic December begins with the Merseyside derby at Goodison Park on the 1st, before trips to Wolves and AC Milan.
Klopp also revealed on Friday that Joe Gomez and Curtis Jones will miss Sunday's game at the London Stadium.
“Curtis is a completely different story, he was very unlucky in training," said the manager, "He got a finger that scratched the eye.
"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.
“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, hopefully after the international break as well.”
Klopp also hopes to have Naby Keita and James Milner back from their individual hamstring issues after the international break.
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