The Kansas City Chiefs starting quarterback left the game during the second quarter after taking a hit from 340-pound defensive tackle Howard Green. Cassel later told The Kansas City Star the team was "just being cautious" after he had the wind knocked out of him and that he wanted to re-enter the game.
It turns out Cassel's condition could be more serious.
Third-year pro Tyler Palko is the No. 2 quarterback behind Cassel. Palko has never started an NFL game.
Both Cassel and Haley maintained after the game that the injury wouldn't keep him out against the Bills.
"He wanted to go back in," Haley said, per The Star. "I thought the prudent thing was to be smart there. ... Unfortunately, he got dinged up a little bit. I'm not overly concerned."
The Associated Press contributed to this report.