Welterweight champion Georges St. Pierre suffered a knee injury that will keep him out of the main event at UFC 137, president Dana White announced Tuesday.
"I just landed in New York City, and what do I always say? I have to deal with bad [stuff] every day," White said on Twitter. "GSP is out with an injury.
"It's [his] knee."
St. Pierre was scheduled to defend his title against hard-hitting Carlos Condit on Oct. 29 at the Mandalay Bay Events Center in Las Vegas.
Firas Zahabi, who is St. Pierre's head trainer, told ESPN.com the fighter's injury isn't career-threatening but severe enough to prevent him participating in strenuous workouts.
"It's not the worst injury, just something that will keep him out the next few weeks," Zahabi said.
"This is not career-threatening; that's the good news. The unfortunate news is that he won't be able to fight. He's really depressed about it."
Though Penn-Diaz becomes the main event, it will remain a three-round fight. UFC recently designated all its main-event bouts for five rounds.
With St. Pierre out, Condit has been pulled from UFC 137. He retains his top-contender status and is expected to challenge St. Pierre at a later date.
"We will sit Condit and wait for GSP," White said. "It's looking so far like [St. Pierre] could be ready in a couple of months."
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.