Georges St. Pierre out with knee injury

For the first time since 2006, the UFC will promote a welterweight championship fight without the considerable star power of Georges St. Pierre.

St. Pierre has been forced from a Feb. 4 title defense in Las Vegas against Nick Diaz after injuring his knee, the second time in less than two months the Canadian has faced such adversity.

On a conference call Wednesday, St. Pierre's doctor said the fighter suffered a completely torn ACL in his right knee and is expected to need 6 to 9 months to recover following surgery.

St. Pierre instead will sit on the sideline as Diaz meets Carlos Condit for an interim UFC welterweight belt, White announced on his Twitter page Wednesday.

Trainer John Danaher said St. Pierre attempted to train through recent injuries to both knees, but that one of the knees felt unstable.

"It did not appear to heal and training suffered," Danaher said in a text message. "He went in for MRI and it clearly revealed (an) ACL tear and MCL damage. We are now weighing surgery options. He is obviously very disappointed -- he is used to training every day, so this long layoff will be difficult to endure."

The news caps a frustrating year for St. Pierre, who's fought just once, a decision victory over Jake Shields in April.

St. Pierre was scheduled to face Diaz in October, but the UFC removed Diaz from the fight for failing to fulfill his promotional duties. Condit was to take Diaz's place, but that bout was sideswiped when St. Pierre's knee gave way while training.

After Diaz defeated B.J. Penn in a revamped main event at UFC 137, he was reinstalled as St. Pierre's next challenger.

Diaz and Condit will now fill the void as headliners on UFC 143, scheduled for Super Bowl weekend at the Mandalay Bay.

Diaz steps into the interim title fight on the strength of 11 straight wins, including an impressive showing against Penn in late October. Condit, meanwhile, has captured 12 of his last 13 fights, and established himself as one of the sternest tests in the UFC welterweight division.

Diaz-Condit will be the second interim UFC welterweight title fight in the division's history. The first came in 2007, when St. Pierre fought Matt Hughes while the sitting champion, Matt Serra, recovered from injury.

Josh Gross is a mixed martial arts writer for ESPN.com.