After the home UEFA Champions League win against FC Midtjylland, Jurgen Klopp was unsure about the seriousness of Fabinho's injury, leaving Liverpool in a precarious position in defence.
The Brazilian came off just 30 minutes into the game, with Liverpool being forced to bring on young Rhys Williams in his place.
This is the latest defensive setback that Liverpool have had to suffer from, with the Brazilian midfielder playing in his unnatural position at centre-back in the absence of Virgil van Dijk and Joel Matip.
Fabinho seemed to pull up whilst running, and then stopping to clutch his hamstring.
When asked about the extent of Fabinho's injury, Klopp said "I don't know. You know, muscle injuries are like this - you get the result, but tomorrow morning probably. So we have to wait for that."
He also added that this is "exactly the last thing we needed. I don't know ... I know he felt a hamstring and that's not good."
Both Van Dijk and Matip suffered injuries in the 2-2 Merseyside Derby draw on 17 October, with the Dutch international suffering an ACL injury that is set to keep him out of action for several months.
Klopp was full of praise for young Williams, with the 19-year-old replacing the Brazilian against Midtjylland, saying "He did really well. The only problem we have now is we have four days to the West Ham game and you will make every day a story of him and then he has to prove he can deal with that as well, because the vibe was calm around him."
As a result of the injuries, Klopp now has to choose between three very young defenders to play in the weekend, with there also being Nathaniel Phillips and Billy Koumetio apart from Rhys Williams, saying "We have very young and inexperienced alternatives, and at the moment it looks like we have to choose one of them."
Liverpool were largely underwhelming against the Danish outfit, with Klopp opting not to start Mohamed Salah, Sadio Mane or Roberto Firmino for the group stage game, yet goals from Diogo Jota and Salah, with the latter coming on in the second half, were enough for the Premier League champions.