Noni Madueke is facing two months out with the knee injury he suffered against Manchester City.
Scans on Monday showed Madueke has not suffered an anterior cruciate ligament injury but the setback for the £52million signing represents a major blow for Arsenal.
The 23-year-old, who suffered the injury in the first half of the 1-1 draw against Manchester City, will miss some key games.
Madueke was replaced by the returning Bukayo Saka on the right wing at half-time and underwent a scan on Monday to assess the damage.
Arsenal are now set to lose one of their in-form attacking options until at least November.
The Gunners face Port Vale in the Carabao Cup on Wednesday and Madueke is set to miss at least 10 games.
Arsenal face Newcastle in a crunch game at St James’ Park on Sunday and Madueke is also set to miss Premier League games against West Ham, Fulham, Crystal Palace, Burnley and Sunderland.
He will also miss Champions League games against Olympiakos, Atletico Madrid and Slavia Prague.
Arsenal will hope he can be back in time for the north London derby on November 23, which is followed by big games against Bayern Munich in the Champions League and then Chelsea.
Madueke was signed to add quality depth to Arsenal’s attack and he has shown his versatility, playing off both flanks in his first few games with his new side.
While Madueke’s injury will be a blow to Arteta, Saka returned in the 1-1 draw with Manchester City when he replaced Madueke at half-time.
But Mikel Arteta will want to ease him back into the team.
Gabriel Martinelli has also demonstrated that he can fill that berth, and fellow new recruit Eberechi Eze has played off the left wing since his arrival from Crystal Palace.
