For now, there are no plans to put Cassel on injured reserve, but when asked if Cassel's season could be cut short due to the injury, Haley said, "That's a possibility. I wouldn't say it's an absolute."
Cassel is thought to have been injured when he was sacked by Denver's Elvis Dumervil and Von Miller, though Haley wasn't sure if it was the hit or the way Cassel landed on the turf that caused it. Cassel wanted to keep playing, but was told by the team's medical staff he was not allowed to return and was replaced by Tyler Palko.
Cassel vowed he would return to start Monday night unless he was "told otherwise," but it appears likely that Palko will be starting against the Patriots.
"We feel good about Tyler, or he wouldn't be here and he wouldn't be our number two," Haley said. "In his case, there's not a lot of regular-season action to go on, but I feel like he's done a great job while he's been here of developing and getting better."
The Associated Press contributed to this report