Citing a source within the organization, Yahoo! Sports reported Cassel could miss two to four weeks. He was hurt Saturday night when sacked on the third play of a 14-10 preseason loss to Seattle.
The Chiefs open the regular season Sept. 13 at Baltimore.
Head coach Todd Haley refused to comment Monday on the status of Cassel.
"I'm only going to talk about the players who are on the field," Haley said.
The top candidates to replace Cassel are Tyler Thigpen and Brodie Croyle, who are 1-18 as starting quarterbacks in the NFL. Thigpen took about 80 percent of the snaps with the first team in practice Monday, but a decision could be based on who plays well in the final preseason game at St. Louis on Thursday night.
"It's almost unheard of that you have something like that happen right now," Thigpen said. "We've got to take responsibility."
The players indicated they had no inkling how long Cassel might be gone.
"It's unfortunate for Matt," Croyle said. "Who knows the timetable when he'll be back? If it comes time to go, I'll just have to be ready."
The fourth preseason game, usually little more than a final tryout for rookie free agents and long-of-tooth veterans straining to stretch their career one more year, could be critical for the Kansas City quarterbacks. Whoever shows well against the Rams could find himself calling plays in Baltimore.
"We've got to have our ducks in a row, no doubt," Haley said. "They're going to kick off in 12 days, or 13 days. We've got to have everything lined up and be ready for anything."
Whoever starts and whoever calls plays is likely to have a tough time until the Chiefs get their offensive line straightened out. In just 19 pass plays in the first three preseason games, Cassel was sacked four times, including the play when he got hurt.
"Until we get on the field and we're winning games and doing the things we need to do detailwise, it doesn't matter who calls the plays," running back Larry Johnson said. "We've got to go out and execute and do the things we need to do right."
This time a year ago, Croyle was designated as the foundation quarterback for Herm Edwards' rebuilding program. But Croyle missed almost the entire season with knee surgery and, when backup Damon Huard was also shelved with an injury, the job fell to Thigpen.
Croyle would certainly welcome the opportunity to win the starting job after spending several months getting over the surgery.
"You've got to be ready for it. This is what I've rehabbed for," he said. "This is what I've tried to get back for."
So far in preseason duty, Croyle has looked the best. In a dismal loss to the Seahawks on Saturday night with Thigpen playing most of the way, the Chiefs were 1-for-10 on third down.
"When you're 1-10 on third down, you're not getting it done at quarterback," Haley said. "There's just no way. Even if there are breakdowns at other places, the quarterback, it's his responsibility to move the chains. At 1-10, there really wasn't much good happening."
Thigpen, a former seventh-round draft choice, started 11 games last year and caused the Chiefs to switch their offense to a spread formation because he had trouble lining up and executing under center. This time last week, Thigpen was also the subject of trade rumors.
"That's the way this business works," said Thigpen, who took most of the snaps with the first team in practice on Monday. "You've got to be ready."
Johnson winces at memories of last year's 2-14 campaign.
"It was tough. We went through three different quarterbacks, then we had to change almost the whole offense to fit one quarterback's strengths. [Chan Gailey] did well for what he was working with. It's not like we were out there with 11 Pro Bowlers.
"To me, it seems like déjà vu five times over. It happened with Trent [Green in 2006], it happened with Brodie, it happened with Damon."
Information from The Associated Press was used in this report.