Liverpool head coach Jurgen Klopp has confirmed Roberto Firmino has sustained a "serious hamstring injury" and is set for a spell on the sidelines.
Firmino was introduced at half-time in the Reds' 2-0 Champions League win over Atletico Madrid at Anfield on Wednesday, but was forced off in the 78th minute.
When the striker will return to action is not clear, but Klopp expects the Brazil international to be out for at least four weeks.
"Bobby is not good news with a serious hamstring injury, really unfortunate," Klopp told Liverpool's official website. "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."
Klopp reiterated the significance of the 30-year-old's absence in his pre-match news conference ahead of a Premier League meeting with West Ham on Sunday:
The German boss said: "Losing Bobby Firmino is a blow. 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."
Roberto Firmino sustained a serious hamstring injury during our win over Atletico Madrid, Jurgen Klopp has confirmed.— Liverpool FC (@LFC) November 5, 2021
Joe Gomez (calf) and Curtis Jones [eye] are also out of the Reds' trip to London, but Klopp expects both to return after the upcoming international break.
Klopp was less sure about Naby Keita and James Milner, but hopes they will be back in action before long as they recover from thigh injuries.
"Curtis is a completely different story [to Firmino], he was very unlucky in training – he got a finger that scratched the eye," Klopp continued to Liverpool's official website.
"I had a lot of injuries to players in my career, that was not [one]. 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.
"We will see [about Keita and Milner]. For them, I have hope for after the international break but I don’t know exactly. We have to wait. That’s two weeks and a few days from now on, so that’s what I would really wish for but I cannot promise that. But that’s what we are working on."