The Gunners suffered just their second loss of the season on Thursday, going down 2-0 at PSV in the Europa League to take the fight for top spot in their group down to the final matchday.
It was a dismal evening for Arsenal, who saw PSV put the ball in the back of the net five times before the offside flag ruled out three.
Mikel Arteta's evening appears to have worsened after Gabriel Magalhaes was seen "limping down the tunnel", and was "grimacing in pain."
It remains unclear how serious the injury is, as manager Mikel Arteta did not address the issue in his post-match press conference in Eindhoven and is not speaking to the media on Friday.
Gabriel has played every minute of Arsenal's Premier League season so far and is now at risk of missing the visit of Forest on Sunday.