Knee injury tough to take - Cruden

New Zealand fly-half Aaron Cruden feels like he has been "struck down by a sniper" after discovering the extent of a knee injury that looks set to keep him out of this autumn's Rugby World Cup.

The 26-year-old Chiefs stand-off ruptured his left anterior cruciate ligament during Friday's 26-9 Super Rugby victory over the Crusaders, when he was running a support line and tried to change direction.

At the time Cruden, who has 37 caps, was not certain how bad it was but later the potential consequences of the knock truly hit home.

"I wasn't too sure what it was when it happened. It wasn't too sore at the time and it is not sore now. When the medics came out I tried to run it off but when I put a little weight on the leg it didn't feel right," he said on New Zealand's official website.

"It was hard to believe when I heard what it was initially. There was no swelling around the knee and it still feels reasonably strong. It felt like I had been struck down by a sniper. It was a pretty tough pill to swallow."

Cruden, who started the All Blacks' victorious 2011 final before having to go off with another knee injury, will now have to undergo a knee reconstruction and he is expected to be out for around six months - with New Zealand's World Cup defence kicking off on September 20.

His absence could give All Blacks coach Steve Hansen a headache as he looks to keep hold of the Webb Ellis Cup, but for now Cruden is just focusing on his recovery.

"I will have to re-evaluate my goals and set other ones and then get into my rehabilitation," he added.