Roy Hodgson fears Dirk Kuyt could be out for longer than the four weeks first anticipated following his shoulder injury, though the unpredictable nature of the problem means the forward could also return ahead of schedule.
Kuyt sustained the injury during training with the Netherlands national side this week and was immediately ruled out for four weeks by Liverpool's medical staff.
While Kuyt said earlier on Wednesday he hopes to be fit to face his former side FC Utrecht in the Europa League on Septemer 30, Hodgson is concerned that the Dutchman may be unavailable for a prolonged period of time.
"It could be more than four weeks," Hodgson said. "It's one of those injuries where it's very difficult for the doctor to say and I think four weeks is a very conservative estimate. If we're lucky it could be earlier, but it could be longer. We have to be very careful because it's the type of injury that has to be treated with a lot of respect.
"It's a great blow for us because Dirk is an important member of our first team. He does a job which is not easy to replace because he has all the qualities required for that role on the field.
"I was very sad to see such a thing happen, especially in a training session. It would have been bad enough to happen playing for the national team in a competitive match, but for it to happen in training was even more of a blow.
"He's a quick mender and he has a great mentality and attitude towards recovery, so he will be back as quick as is humanly possible. But we're looking at least a month and possibly longer."
However, Liverpool do expect young striker David Ngog to be available for the weekend's game at Birmingham City despite being withdrawn from the France Under-21 squad due to a hamstring injury.
"He has been nursing a problem but is much better now," Hodgson said. "We were obliged to withdraw him from the French Under-21 squad because when they got together ten days ago he wasn't fit.
"The French people were very good about that and realised there was a problem. I am hoping that he will recover in time for this weekend. We have nursed him through and hopefully he will train either today or Friday."
Hodgson also delivered a positive bulletin on the fitness of Brazilian full back Fabio Aurelio, who is currently unavailable due to an Achilles problem.
"Fabio is making very good progress and is definitely moving in the right direction," Hodgson said. "It was a relatively minor problem but one we had to nip in the bud because Achilles injuries can be very problematic."