Mack, a Pro Bowler, was placed on injured reserve Friday after having surgery Thursday to repair a broken left fibula. The injury occurred during Sunday's win over the Pittsburgh Steelers.
The leg broke when Mack's left foot twisted 90 degrees from the way his knee was pointed as players fell in a pile at the end of a play. The torque of the blow caused the break, and potentially also caused ligament damage.
The team did not release any information about ligament problems, but coach Mike Pettine had said Mack might be able to return if the only issue was the broken fibula. Ligament damage seems to be the "something more" that Pettine said would end Mack's season. ESPN injury analyst Stephania Bell said that ligament damage "is not uncommon" in an injury like Mack's.
Mack did not miss a snap in his six seasons with the Browns until the injury -- a streak of 5,279 plays. He went to the Pro Bowl last season and was probably playing at a higher level this season.
"It's a huge blow," offensive coordinator Kyle Shanahan said Thursday. "It's just part of the business, but Alex is as good a football player as I've been around. We're going to miss him a lot, and you don't replace a guy like that. He's too special of a player."