The England international was nowhere to be seen on Liverpool's teamsheet for Tuesday's Champions League clash with Porto after reports on Monday claimed that he had not travelled with the squad to the Estadio do Dragao.
Alexander-Arnold was originally reported to be nursing a minor injury, and Klopp has now confirmed that a muscular issue could put his participation in Sunday's game in doubt.
Speaking to BT Sport, Klopp said: "Unfortunately he is injured. He came in from training and we did further assessment and it looked [more] serious. It's a muscle, it's a bit of tightness. He's out for today and it doesn't look great for the City game either."body check tags ::