A knee injury suffered in training Tuesday has forced welterweight champion Georges St-Pierre out of next week's UFC 137 title fight with Carlos (Natural Born Killer) Condit.

"We don't play golf," a muted St-Pierre told The Canadian Press. "It's a full-contact sport. This stuff happens."

St-Pierre sustained a strained knee ligament defending a takedown at Montreal's Tristar Gym in his final week of training. The champion says he will be sidelined about four weeks.

The 30-year-old from Montreal was slated to meet Condit on Oct. 29 in Las Vegas. St-Pierre was supposed to wrap up training camp Saturday.

The new marquee bout at the Mandalay Bay Events Center will be a three-round welterweight bout between B.J. Penn and Nick Diaz, which had been the co-main event.

The card is already sold out, but the UFC will doubtless feel the financial pinch in its pay-per-view audience. UFC president Dana White, who said via Twitter he is "beyond" devastated at the news, has said St-Pierre is the organization's pay-per-view king and the fighter's loss may well deter viewers from digging into their wallet.

Ironically, St-Pierre was originally slated to meet Diaz in the main event, but White replaced Diaz with Condit after the California challenger, who does not play well with others when it comes to media, failed to show for publicity events in Toronto and Las Vegas.

White then pitted Diaz against Penn.

Welterweight Josh Koscheck, no stranger to stepping up to take fights on short notice, promptly offered Tuesday to fight in place of St-Pierre.

"Duty calls again???" he said in a tweet to White.

But Condit (27-5), who was doing interviews about the GSP fight Tuesday, will still get his shot.

"We will sit Carlos to wait for GSP," wrote White, who tweeted that he respected Koscheck for making the offer. "It's looking so far like he could be ready in a couple months."

St-Pierre (22-2) said he had tweaked his other knee in training last week. It turned out to be just a slight bruise but he believes Tuesday's injury occurred because he was compensating for the other knee.

"It's just the way I tried to defend the takedown," he explained. "I fell down and when I stood up, I could feel something weird."

Trainer Firas Zahabi said St-Pierre initially tried to continue.

"After the takedown it looked a little weird," he said. "I was actually a little concerned but I wasn't sure how bad it was.

"Once he started moving around, I saw that he was limping and he had to stop."

Zahabi says St-Pierre needs a month off — physically and mentally.

"The [training] camp was extremely rigorous and I think just mentally he needs some time and he needs to get ready for the next training camp," he said.

With high-profile main events already booked in cards through mid-December, St-Pierre has some time to retool.

"I don't know what their [UFC] agenda looks like but I know Georges needs a month to heal and then two months to prepare," Zahabi said.

Condit, a former WEC champion, said in a statement that while he was "greatly disappointed," he understood "injuries are a part of this business."

"Georges is a great competitor and I know that he will want to return to 100 per cent health as soon as possible so that this fight can be rescheduled," he added.

"This is the best training camp I have ever had, and I look forward to carrying over the growth that I've experienced in this camp to my next one.

"I have trained for over a decade in mixed martial arts with the clear goal of becoming the best fighter in the world at my weight class. I have worked this long to become the UFC welterweight champion. I will work a little longer."

The UFC has been hit hard by injuries in recent times, reflecting a hard-nosed sport where the training can be tougher than the fights themselves.

Lightweight champion Frank Edgar's recent title defence against Gray Maynard was delayed when both fighters were injured in the leadup. Heavyweight title-holder Cain Velasquez has been sidelined by shoulder surgery.

Light-heavyweight champion Jon (Bones) Jones, former light-heavyweight title-holder Rashad Evans and former heavyweight champ Brock Lesnar (diverticulitis) have also been sidelined for varying amounts of time.

It has made for some frantic rejigging of cards.

In Diaz and Penn, the UFC has a pair of name fighters as a replacement main event — although not with the same star power of GSP.

Diaz (25-7-1) is making his return to the UFC after a stint as Strikeforce champion.

Penn (16-7-2) is a former UFC lightweight and welterweight title-holder who has gone 1-2-1 in his last four fights.

He lost his 155-pound title and a rematch to Frankie (The Answer) Edgar last year before knocking out former 170-pound champion Matt Hughes.

He fought welterweight Jon Fitch to a tie at UFC 127 in February.

Penn and St-Pierre have been linked by injury before.

St-Pierre was slated to meet champion Matt Hughes at UFC 63 in September 2006 but had to withdraw due to a groin injury. Penn stepped in for the Canadian, losing by third-round TKO.

St-Pierre went on to beat Hughes for the championship at UFC 65 in November 2006. He lost the title next time out to Matt Serra at UFC 69 in April 2007 — a fight delayed two months due to a GSP knee issue — but won it back from Serra at UFC 83 in April 2008.

The Canadian has defended the welterweight championship six times since.

St-Pierre also tore his abductor muscle — in three places — in beating Thiago Alves at UFC 100 in July 2009.

Zahabi says he sees beyond the disappointment of this latest injury setback.

"I like to think the training is still there. it still counted for something. There's going to be another fight … He's improved, he's come a long way, he's progressed as he does in every camp.

"I was very happy with the camp."