Arsenal could be without Rob Holding for up to nine months after scans confirmed the central defender ruptured the anterior cruciate ligament in his left knee against Manchester United.
The 23-year-old suffered the injury in the 'Gunners' 2-2 draw against Manchester United at Old Trafford.
"Rob will undergo surgery in the coming days and rehabilitation is expected to take between six to nine months," said an Arsenal statement.
"Everyone at the club will now be working as hard as we can to ensure Rob is back on the pitch as soon as possible."
The former Bolton Wanderers defender also thanked Arsenal staff and supporters for their support on Instagram.
"Just want to say a big thank you to all the fans, players and staff members for the support during the past couple of days.
"Gutted to get injured during a strong period for the team!"
The injury is the latest defensive setback for Arsenal, who remain without centre-back Laurent Koscielny and left-back Nacho Monreal