Arsenal have confirmed defender Calum Chambers ruptured the anterior cruciate ligament of his left knee in Sunday's defeat by Chelsea.
Chambers, 24, had successful surgery on Thursday and is expected to be out for action for six to nine months.
The centre-back, who has made 18 appearances in all competitions this season, suffered the injury in the first half at Emirates Stadium.
Gunners boss Mikel Arteta previously said the injury was "significant".
Meanwhile, Arsenal hope left-back Kieran Tierney will return to training in March after undergoing surgery on the dislocated shoulder he sustained in the 3-1 win over West Ham on 9 December.
Arteta, who took charge last month, hopes former captain Granit Xhaka will stay with the club, having been linked with a move to Hertha Berlin this month.
Swiss international Xhaka, who joined Arsenal from Borussia Monchengladbach in 2016, impressed in Wednesday's 2-0 Premier League win over Manchester United.
Two months earlier, he had lost the captaincy after reacting angrily to boos from the Arsenal crowd following his substitution against Crystal Palace at the Emirates.
"I wanted his team-mates to get his back and help him to change his mind because we need him," Arteta said.
"I believe that if we were able to do that the fans would respond in a positive way.
"I've seen him act every day, how he trains, how he lives his profession and the quality of the player. He could be a really important player for the club."