All Blacks and Chiefs first five-eighth Aaron Cruden is understood to have suffered a serious knee injury which could threaten his Rugby World Cup participation.

A grimacing Cruden limped from the field midway through the first half of the Chiefs' 26-9 Super Rugby victory over the Crusaders in Christchurch on Friday night after receiving attention to his left knee from Chiefs team doctor Kevin Bell for some time out in the middle of the park.

Chiefs head coach Dave Rennie was reluctant to comment on Sunday, saying everyone would likely know officially what the situation was on Monday. Cruden was due to undergo a knee scan on Monday.



It is understood New Zealand Rugby is planning to make an announcement on Cruden's immediate future but no-one was prepared to comment ahead of that.



If Cruden has ruptured his anterior cruciate ligament or done damage just as serious to his knee then that would put him out for six to eight months and therefore put him out of the All Blacks' 2015 campaign, including the World Cup in Britain.



All Rennie would confirm was that Cruden was definitely out of this Friday night's Chiefs match against the Force at Waikato Stadium in Hamilton.

While Damian McKenzie, Andrew Horrell and Marty McKenzie are all contenders to take over from Cruden in the No10 jersey for the Chiefs, if his injury is serious then others will also move up the pecking order for the World Cup.

Luckily the All Blacks have an embarrassment of riches in the pivot position with Dan Carter, Beauden Barrett and Colin Slade all established in the national side and experienced in the white-hot heat of test rugby, while a matured Lima Sopoaga has been a standout in Super Rugby for his high-flying Highlanders side this season.

Rennie said the Chiefs would clarify who would fill the first-five spot for them this week in the next few days.

"We've got Marty McKenzie who steered a team [Taranaki] to an ITM Cup last year, we've got Damian who we've used already this season and we've got [Andrew Horrell] who is a jack of all trades who has played all over the place for us over the years and is a smart footballer who could do the job for us too," Rennie said.

Horrell came off the bench to replace Cruden on Friday night with Damian McKenzie rested from the match 23 for the first time in a number of games this season.

The Chiefs are in otherwise good shape for their two remaining matches against Australian opposition - the Force and the Rebels - before their second bye.

"James Lowe got a bit of a knock and we've got quite a few bumps and bruises as you can imagine from what was a pretty brutal game, but at this stage we don't actually think it will affect selection this week and other than Aaron we should be OK."

The Chiefs, having played one extra game, sit just one point behind the New Zealand Conference and overall competition-leading Hurricanes, who suffered their first loss of the season when they went down to the Waratahs on Saturday night in Wellington but still picked up two bonus points.

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The Highlanders have also played one less game and after their win over the Blues on Saturday they still sit just four points adrift of the Chiefs. The number of games played will start to even up once the Chiefs have their second bye.

The Chiefs have traditionally struggled against Australian opposition, although they did beat the Brumbies in New Plymouth earlier this season 19-17.

The Force have won just one match this season and the danger is the Chiefs will subconsciously ease up after such a massive effort against their traditional rivals, the Crusaders.

"We always get up for the Crusaders who are chocker full of All Blacks and we need to be able to bottle that up and play with the same intensity each week.

"Playing the Force this week with it being Anzac Weekend, it means a hell of a lot to both countries and so we'll tap into a little bit of that as well.

"But it's very much about personal accountability and working out how they got themselves into that sort of mental state to play with that sort of intensity last week and replicate it week-in, week-out."

The Force beat the Chiefs in Perth last year and Rennie said the in-form Rebels, who beat the Brumbies at the weekend, would also present a big challenge the following week in Melbourne.

Meanwhile Rennie was effusive in his praise of the unsung Chiefs forward pack where his tight five were outstanding against an All Blacks-laden Crusaders unit and openside flanker Sam Cane a shining light in the loose.

"I thought he was phenomenal - the amount of ball he stole, the ball he slowed down for them so we could set our defence, the amount of tackles he made - 15 out of 15 - and he was a real menace around the park.

"He also carried well for us and his continuity play was great and he played a big part in our success at the weekend," Rennie said.

All Blacks lock Brodie Retallick shone in an impressive tight forward group, while tighthead prop Ben Tameifuna anchored the Chiefs' superior scrum, made some huge hits on defence and cleaned out and carried strongly.

While the Chiefs gave away nine first-half penalties, including one for obstruction on attack that ruled out what would have been the opening try, that was reduced to just three in the second spell and Rennie felt overall their discipline continued to improve.