Mikel Arteta said Bukayo Saka's latest injury is "a worry" following Arsenal's 2-1 Champions League defeat at Lens.
Saka, who set up Gabriel Jesus' goal, went off with a muscular problem in the 34th minute, five days before a crunch Premier League game with champions Manchester City at Emirates Stadium.
Arteta did not reveal how long he thinks the 22-year-old could be out for, but told TNT Sports: "It was a backheel, an action that can produce that kind of injury.
"He tried to backheel a ball in the first half and felt something muscular.
"He felt uncomfortable to carry on so we had to take him off.
"He hasn't really missed games. We gave him a break against Brentford [in the Carabao Cup] last week and that was all."
Saka's injury is a massive blow to Arsenal as so much of their attacking play goes through the England forward, who has started 87 consecutive league games.
Arteta is now hoping not to lose him for the visit of Pep Guardiola's City, who lead the Premier League and are one point clear of Tottenham and Arsenal.
"Let's see what the extent of it is," he added.
"The last few were more knocks than anything else."