Liverpool centre-back Virgil van Dijk is to see a specialist to assess the extent of a hamstring injury.
The Netherlands defender, who has played in every Liverpool league game this season, was substituted at half-time in Monday's defeat at Brentford.
There is no timescale on how long Van Dijk will be out for but the injury appears worse than originally thought.
The Reds' other centre-back options are Joel Matip, Ibrahima Konate, Joe Gomez and Nathaniel Phillips.
Liverpool, who are sixth in the Premier League and seven points off the top four, face Wolverhampton Wanderers in the FA Cup third round on Saturday.