Sir John Kirwan believes Aaron Cruden is the answer to the All Blacks' World Cup hole left by injured utility back Damian McKenzie.

McKenzie's season-ending injury, a ruptured ACL in his right knee suffered in the Chiefs' win over the Blues last week, has likely ruled him out of the World Cup, leaving Steve Hansen and the All Blacks selectors with a major headache.

The 23-year-old's ability to play at both No10 and fullback made him the perfect back-up for All Blacks first-fives Beauden Barrett and Richie Mo'unga.

The injury threatens to significantly test the All Blacks' depth at first-five, with several inexperienced names like Blues' Otere Black, Crusaders' Brett Cameron and McKenzie's brother Marty being touted as potential replacements.

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However, All Blacks great Kirwan believes Hansen and co should look elsewhere, more specifically to France.

Speaking on Sky Sport's The Breakdown, Kirwan said All Blacks selectors should consider an attempt to bring Cruden back from France to help them defend the World Cup in Japan.

"I think Cruden's the best," Kirwan said. "His club doesn't like him, but he's got to come home now.

"He's got to sign with New Zealand Rugby, come home for next year and start playing."

The 50-test All Black has fallen out of favour at his Montpellier club after a string of injuries saw him sidelined for the majority of the Top 14 club's fixtures last season.

Cruden, who signed a four-year deal worth $3.6 million with Montpellier until 2020, was a member of the All Blacks side that won the World Cup in New Zealand in 2011.