Danny Welbeck suffered a "significant right ankle injury" during the goalless Europa League draw with Sporting CP on Thursday, Arsenal have confirmed.
England international Welbeck received oxygen on the pitch and was taken straight to hospital after landing awkwardly following a header during the first half at Emirates Stadium.
The Gunners are yet to confirm the exact nature of the issue, although it appears the forward is set to miss several months.
An Arsenal statement read: "Danny remains in hospital and will continue to be assessed. A further update will be issued over the next 72 hours."
The injury casts Welbeck's club future into further doubt, with the 27-year-old out of contract at the end of the season.