Liverpool midfielder Arthur is facing up to four months out of action.
Jurgen Klopp confirmed that the deadline day signing had suffered an injury in the build-up to his side’s Champions League win over Rangers on Tuesday.
The Brazil international has played just 14 competitive minutes for the Reds this season, with his only prior involvement being with their under-21 side.
Liverpool had hoped Arthur would supplement their midfield options following his season-long loan arrival from Juventus on the final day of the window.
But Anfield officials now estimate that the former Barcelona playmaker, 26, will be sidelined for between three and four months with a muscle tear.
Arthur last featured for the Serie A outfit in their Coppa Italia final defeat to Inter Milan in mid-May before being deemed surplus to requirement.
“We have one new injury, it’s Arthur,” said Klopp on Friday.
“In the last session before the Rangers game, in a normal situation he just injured a muscle.
“We have to wait for further assessment to know exactly how long it will be but he’s out.”
Arthur’s representative recently refuted claims that the player was set to be sent back to Italy early from the loan spell amid doubts over his abilities.