Trent Alexander-Arnold has suffered a groin injury with Liverpool hopeful the defender can avoid an extended spell on the sidelines.
Alexander-Arnold didn't travel with the squad that flew out to Porto for the Champions League group encounter on Tuesday evening.
The right-back had been spotted training normally on Monday afternoon ahead of the journey during the part of the session that was open to the media.
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However, it is understood Alexander-Arnold sustained a sore groin later on with a decision swiftly taken to rule him out of the trip to Portugal.
And the defender is now almost certain to miss the Anfield clash with champions Manchester City on Sunday and is expected to be ruled out of England's forthcoming World Cup qualifiers against Andorra and Hungary.
Liverpool will hope Alexander-Arnold isn't absent for any significant period given another busy run after the international break of seven games in 23 days, starting with successive away games at Watford in the Premier League, Atletico Madrid in the Champions League and a short trip to Manchester United.
Jurgen Klopp has a number of right-back options at his disposal should Alexander-Arnold be out for any greater length of time.
James Milner, who deputised for an ill Alexander-Arnold in the 3-0 Premier League win over Crystal Palace earlier this month, started at right-back in Porto while Wales international Neco Williams was on the bench after a month out through injury.
Joe Gomez has previously played at right-back while 18-year-old Conor Bradley impressed on his debut in the position in the Carabao Cup win at Norwich City last week.
