Roberto Firmino has suffered a "serious" hamstring injury and is unlikely to return after the international break next week.
Firmino limped off a substitute in Wednesday's Champions League match against Atletico Madrid.
"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."