Georges St. Pierre off UFC 137, Diaz-Penn to lead card

Welcome back to the Ultimate Fighting Championship's main event, Nick Diaz.

A knee injury to welterweight champion Georges St. Pierre has forced him out of UFC 137 on Oct. 29 in Las Vegas, UFC President Dana White announced Tuesday. The co-main event of Nick Diaz vs. BJ Penn will move to the top of the card, White said.

St. Pierre was supposed to fight Carlos Condit for the 170-pound title. Now the switch means Diaz returns to the headline slot that was originally his before UFC pulled him from atop the card after he missed press conferences to promote the event.

The champion suffered a "bad sprain" during a sparring session on Tuesday, trainer Firas Zahabi told Sherdog.com.

Of 16 shows for UFC since mid-March, this marks the seventh instance of a change in the main event because of injury or illness.

Despite the loss of St. Pierre, UFC 137 will still have its share of star power. Penn's titles in multiple weight classes and complex skill-set have established him as one of the sport's all-time greats in the cage. Diaz has been arguably MMA's most volatile personality at the elite level, a man whose aggression in the cage is matched by his outbursts of anger outside it.

Diaz and Penn are Nos. 4 and 7 in the USA TODAY/MMA Nation consensus rankings for welterweights.

The duration of St. Pierre's layoff remains unknown, White said, adding that the champion might be ready to return within two months. Rather than pursue another fight for Condit, UFC will keep him on the sidelines until he can face St. Pierre, White said.

"I am greatly disappointed, for both myself and UFC fans," Condit said in a statement. "But I also understand that injuries are a part of this business."