Posted by ESPN.com's Bill Williamson
Cassel was hurt when he was sacked early in the game against Seattle on Saturday.
The Chiefs start the season Sept. 13 at Baltimore. It may be difficult for Cassel to heal and get enough practice time in before that game.
Croyle seemed to be a better fit for new coach Todd Haley's system than Thigpen, who is more of a freelancer.
Cassel's injury is a major disappointment for the Chiefs. The team has centered its franchise on Cassel, who was given a contract extension that will pay him $40 million in the next three years.
The injury is also concerning for the Chiefs because it further illustrates the Chiefs' problems on the offensive line. Last year, both Croyle and Damon Huard were lost for the season.
Cassel had been on the run most of the preseason before being hurt on the sack.
Now, it looks like the Chiefs may have to turn to a backup quarterback once again.