Alexander-Arnold set up the opener for Diogo Jota on Saturday with a pinpoint corner, but did not return for the second half with the score at 1-1.
Immediately that sparked fears of an injury blow for Alexander-Arnold, whose left knee was strapped before kickoff – and that has now been confirmed.
Live coverage of the fixture explained that the substitution came as an injury precaution, with the assumption being that it was to the playmaker’s knee.
It comes just over a month after Alexander-Arnold suffered ligament damage in the same knee that had strapping on it on Saturday.
Even then, he was not expected to start, with Conor Bradley‘s absence forcing Jurgen Klopp‘s hand, and meaning Alexander-Arnold was subject to a pre-planned substitution just before the hour mark in north London.
His withdrawal a week later does not appear to have been part of the plan, particularly as there was no established cover on the bench.
Joe Gomez missed the game due to illness, while Bradley is back in Northern Ireland as he mourns the death of his father.
Klopp is likely to provide an update on Alexander-Arnold’s situation at full-time, though the club may be required to await the results of a scan to determine any extent of the injury.