"The Chiefs said Matt Cassel had nothing more than the wind knocked out of him when he left their final preseason game Thursday against Green Bay after being sacked midway through the second quarter.

It turns out Cassel has more than that. He has a rib injury that could prevent him from playing in Sunday's regular-season opener against Buffalo at Arrowhead Stadium. If Cassel can't play against the Bills, Tyler Palko is the likely starting quarterback.

Palko has bounced around with several NFL teams but has never started a regular-season game. A source close to the situation said Chiefs coach Todd Haley told Palko after Thursday's game, "You'd better be ready to play Sunday."

Cassel was injured when Green Bay's 340-pound defensive lineman Howard Green landed on top of him after the sack. Cassel got up in obvious pain after the play and immediately left the game in favor of Palko.

Cassel later did some throwing on the sideline as if he was going back in the game, but he never did. Cassel went with the rest of the Chiefs to the locker room at halftime but did not return to begin the second half. He did return to the sideline at the start of the fourth quarter.

After the game, Cassel said he would be ready to play against the Bills.

"I just got the wind knocked out of me," he said. "We were just being cautious.""