Arsenal winger Bukayo Saka was substituted after sustaining a thigh injury in the 10th minute of their 2-2 Champions League draw with Bayern Munich on Wednesday.
The England winger went down after playing a pass from the right side and looked to be in immediate discomfort as Mikel Arteta motioned to his substitutes' bench.
Saka, who could be seen grabbing his left thigh after going down, was replaced by Eberechi Eze with the match level at 0-0 and went straight down the tunnel to the dressing room.
"They are a bit concerned," Artea said after the game, while confirming the injury was to Saka's quad. "A player who has to come off with a really sharp pain is never good news but we have to wait and see."
Arteta added in his news conference: "It doesn't look good."
The 25-year-old Saka has suffered a slew of injuries in recent years, including issues with his hamstring and Achilles last season.
Arsenal were looking to extend their perfect start to the League Phase and minutes after coming on, Eze fired a first-time volley past Bayern keeper Manuel Neuer to give the Gunners the lead.
Alphonso Davies levelled for Bayern soon after Eze's goal and Harry Kane put the hosts ahead from the penalty spot before Declan Rice struck for Arsenal to split the points on the night.
Information from AP and ESPN Global Research was used in this report.
