The bone in the left-footed kicker's knee was broken nearly in half during a morning workout Sunday, coach George Seifert said.
Kasay, a Pro Bowler in 1996, was taken from training camp at Wofford College to Charlotte, N.C., for surgery.
It was Kasay's second practice with the team since returning from surgery to repair a torn anterior cruciate ligament in the same knee. That injury came in a December game, and Seifert wasn't sure if the two injuries are related.
On Sunday, Kasay was attempting a 30-yard field goal when, as his foot connected with the ball, he screamed: "Oh no! No! Trainer! Trainer!" The ball squirted away off to the left.
"It was hard for me to see that much," said punter Ken Walter, who holds for field goals. "I was concentrating on the hold, but I kind of felt something that wasn't normal. He didn't get a good foot on the ball. Next thing you know, I hear him yelling and squirming around on the ground."
Kasay's 37 field goals in 1996 set an NFL record, while his 145 points that year are the third-most ever by a kicker.
Kasay, 30, signed with expansion Carolina as an unrestricted free agent in 1995, after four seasons with Seattle.
He has made 126 of 155 field goals and 154 of 158 extra points with the Panthers.
The injury leaves rookie Eric Meng, from Georgia Southern, as the only kicker on the roster. And Meng who, like Kasay, is left-footed was brought in chiefly because the team wanted a lefty for special teams drills until Kasay returned.
"Meng will have to continue to compete and see if he can hold on to the job or if we'll have to go elsewhere," Seifert said.
Kasay practiced with the team Saturday for the first time since the ACL surgery.
He spent other time in training camp in private workouts on an empty field near the rest of the team. His field goal attempts were sailing 50 yards and his kickoffs traveled deep into the end zone.
"I absolutely feel terrible for him," quarterback Steve Beuerlein said.
"He put so much into his rehab and worked so hard to get back. To have something like this happen, it must be devastating for him. Obviously, it's pretty major for the team as well."
Kasay's injury is only the latest for the Panthers.
Starting wide receiver Patrick Jeffers injured his right knee in Thursday's 13-0 exhibition loss to Pittsburgh, and backu safety Tony Booth also could be gone for the season after re-injuring his surgically repaired left knee in the same game. Second-round draft pick Deon Grant is out for the season with a fractured left hip.
"We've got a black cat for a mascot, huh? I'm starting to wonder about it," cornerback Eric Davis said.
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