UFC president Dana White broke the news earlier in the day via Twitter.
According to White, who spoke to MMAFighting.com via text message after making the announcement, Nick Diaz vs. BJ Penn will now main event next week's pay-per-view event from Las Vegas. The new main event will remain a three-round affair because, according to White, "they haven't trained for five rounds."
Carlos Condit has been removed from the card, and according to Condit's manager Malki Kawa, Condit will receive his title shot against GSP as soon as the champion is cleared to fight again.
"He's out for a month," Zahabi said of GSP. "I would say at least a month he can't train. So unfortunately, he's out of the fight, that's for sure."
"He's not in any condition to fight. No doctor will clear him."
Zahabi said GSP was on his way to get an MRI on his injured knee, and while he hoped GSP would be healthy enough to train again in a month, he was still unsure of the extent of the injury.
UFC 137 takes place Oct. 29 from the Mandalay Bay Events Center in Las Vegas. The main card will air live on pay-per-view.