Cassel won't be placed on the injured reserve list for now, but Haley said the injury was "significant" and could require surgery, The Kansas City Star reported.
Asked if Cassel was likely done for the year, the coach said, "I wouldn't say it's an absolute."
The signal-caller hurt his throwing hand during Kansas City's 17-10 loss to Denver Sunday, which dropped the Chiefs to 4-5.
Backup Tyler Palko, who has seen little action in his two years with the Chiefs, will take over behind center in Cassel's absence.
"We feel good about Tyler, or he wouldn't be here and he wouldn't be our No. 2," Haley told the Star. "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."