Arsenal's 2-0 win over Bayern Munich in the Champions League was marred by a suspected injury to Aaron Ramsey.
The Wales international left the park clutching his hamstring in the 10th minute, replaced by Alex Oxlade-Chamberlain.
"He's alright but very down. He told me he injured his hamstring. It's quite a tough one," Gunners boss Arsene Wenger told reporters after the match.
Ramsey is now expected to spend some time on the sidelines, but it is unclear how long he is likely to be out for.
The Gunners are next in action this weekend when they take on Everton in the Premier League.