Liverpool centre-back Virgil van Dijk is to see a specialist to assess the extent of a hamstring injury.
There is no timescale on how long Van Dijk will be out for but the injury appears worse than originally thought.
Liverpool, sixth in the Premier League and seven points off the top four, face Wolverhampton Wanderers in the FA Cup third round on Saturday.
The Reds' other centre-back options are Joel Matip, Ibrahima Konate, Joe Gomez and Nathaniel Phillips.
After the defeat by Brentford, Liverpool manager Jurgen Klopp said: "Virgil felt a little bit the muscle and said it is fine, and he is a very good judge of these kind of things.