Joel Matip has yet to train with his teammates ahead of Sunday's crunch encounter between Liverpool and Manchester United.
The Red Devils leapfrogged the Premier League title holders into first place on Tuesday night with a 1-0 win over Burnley. The pair meet at Anfield on Sunday afternoon in what is being billed as a potentially defining game for both sides.
Defending champions Liverpool hope to equal United's tally of 20 league titles this season, while the Red Devils have not been top at this stage of a Premier League campaign since 2012/13.
Jurgen Klopp will clearly be keen to have as many players available as possible for the derby clash, but Matip remains a doubt with just days to go.
As reported by the Mirror, Matip has returned to take part in 'specifically-prepared sessions with physios' and is 'running on grass' as he builds up fitness following an abductor strain suffered in the 1-1 draw with West Brom on Boxing Day. However, he is yet to take part in sessions with his Liverpool teammates.
The report claims Klopp 'would need' Matip to come through two group training sessions with no problems to consider him available for selection.
Matip has, of course, been Liverpool's only real senior centre-back option for much of this season, after both Virgil van Dijk and Joe Gomez suffered serious knee injuries last year.
Fabinho has deputised at the back with aplomb but the potential absence of Matip creates a selection dilemma for Klopp ahead of arguably his biggest league game of the season so far.
Rhys Williams (19) and Nat Phillips (23) have played a total of 390 Premier League minutes between them, while either would be seen as a risk - if an unavoidable one given the circumstances - against a side as dangerous as United.
Aside from Matip, the Reds still have Kostas Tsimikas and Diogo Jota ruled out for the match, as well as Gomez and Van Dijk, while Naby Keita is another doubt.