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Alexander-Arnold did not travel with the rest of the Liverpool squad for Wednesday's (AEST) Champions League win over with Porto, James Milner instead taking over at right-back for the Group B fixture.

The England international is also a major doubt for the upcoming clash against the reigning Premier League champions too, with the Reds hosting Pep Guardiola's side at Anfield.

Asked in his pre-match interview if Alexander-Arnold was out due to injury, Klopp replied: "Unfortunately. 

"it was shortly after training. Trent went in a little bit early, but it didn't look serious. 

"Then we did a further assessment and it was serious, so he is out for today, he didn't travel with us, and it doesn't look great for the City game as well. 

"It's a muscle thing. We thought he was a bit tired from the game, but it was more. Now we have to deal with that."

While Klopp refused to divulge any details, the club website confirmed it to be an adductor issue for Alexander-Arnold, who played the entire match as Liverpool drew 3-3 with Brentford at the weekend.

Milner would be the most likely candidate to deputise against City - one of his former clubs - having also filled in on the right side of the defence for the 3-0 win over Crystal Palace earlier this season.

City won 4-1 when they travelled to Anfield last season, emphatically ending a run of 17 away league games without a win at Liverpool.