Athletes from the Ultimate Fighting Championship are known for the brashness and their toughness, but as tough as they may be, there are some risks that just aren't worth taking.

UFC Welterweight champion Georges St Pierre found this out the hard way when he sustained a knee injury in training camp and was forced to withdraw from UFC 137 on pay-per-view Oct. 29.

St Pierre, known for his mastery of several martial arts including Brazilian jujitsu, Kyokushin, Gaidojutsu and wrestling, was scheduled to defend his title against Carlos Condit. After experiencing the injury last Saturday while he was in Montreal, he was reluctant to withdraw, but ultimately removed himself from the main card event. UFC chairman Lorenzo Fertitta says that St-Pierre is expected to be out for four-to-six weeks.

I just land in NYC and what do I always say!? I have to deal with bad shit everyday!! GSP is out with an injury!!! #oneofthosedays, Dana White tweeted late Tuesday evening.

St-Pierre is one of the most respected men in all of mixed-martial arts, and his absence will definitely be felt during the event. He is ranked as one of the best pound-for-pound fighters in the world by several of the sport's premier writers, and he's one of the best selling pay-per-view fighters in UFC history.

The champion has won nine consecutive fights, which is nearly impossible in a sport as volatile as mixed-martial arts. What may be most impressive is that St. Pierre has held the title for so long in the talent-loaded welterweight division. His last loss was in April 2007 against Matt Sera, a fighter who he eventually avenged his loss to. The opponents he's faced during his win streak are among some of the best in the sport, including Matt Hughes and B.J. Penn.

As a replacement for the fight, B.J. Penn will take on Nick Diaz in the main event; meanwhile, Carlos Condit, St. Pierre's original opponent, will be removed from the event all together while he waits for the champion to heal.

It's GSP's knee. We will sit Carlos to wait for GSP. It's looking so far like he could be ready in a couple months. Still early to tell tho, White tweeted moments after he broke the news.

He reiterated in a seperate tweet by writing, Nick Diaz vs BJ Penn is the main event. He wrote one final tweet on the matter: It will be 3 rounds not 5.

Whether the replacement fight will be able to live up to the original main event is yet to be determined, but the Penn-Diaz fight is sure to be exciting due to their similar style. Both men specialize in Brazilian jiujitsu and boxing.