Arsenal full-back Sead Kolasinac "suffered a significant right shoulder injury" during Sunday's 3-2 win against Everton.
The Bosnia-Herzegovina international came off after 18 minutes and Arsenal say he will "undergo further specialist assessments".
Kolasinac, 26, has played 20 games for the Gunners this season.
Arsenal host Olympiakos on Thursday, taking a 1-0 lead into the second leg of the Europa League last-32 tie.
Bukayo Saka, 18, who replaced Kolasnic on Sunday, is expected to start at the Emirates.
Injured full-backs Kieran Tierney and Cedric Soares are expected to return to training by the end of the month.