Arsenal manager Arsene Wenger has said midfielder Francis Coquelin is facing three months on the sidelines.
Coquelin, 24, suffered a knee injury in Arsenal's 2-1 loss at West Brom on Saturday, and Wenger said on Monday he was facing at least two months out.
Speaking after the Gunners' 3-0 win over Dinamo Zagreb, Wenger told reporters the Frenchman will be absent for 12 weeks.
"It is what we feared basically," Wenger said. "It means it will be, let's be realistic -- a medial knee ligament -- 12 weeks."
Coquelin has emerged as a key player for the Gunners since being recalled from his loan at Charlton in December 2014.
Mathieu Flamini started in place of Coquelin on Tuesday night in the Champions League as the Gunners kept their hopes of reaching the knockout stage intact.
Alexis Sanchez scored a brace while Mesut Ozil was also on target ahead of Arsenal's final group game at Olympiakos on Dec. 9.
Meanwhile, Jack Wilshere has said his recovery is going well but he cannot guarantee he will be back at Christmas.