New Zealand's lack of a "Plan B" could derail their hopes of defending the Rugby World Cup, particularly if injury strikes veteran fly-half Dan Carter. That's the opinion of Gain Line founder and former Wallabies prop Ben Darwin, whose company has taken a different approach in trying to understand why teams are successful by measuring areas such as "cohesion" and players' "shared history".

Darwin spoke with ESPN earlier this year, provoking the ire of All Blacks fans after saying the world champions were "on the decline"; something Gain Line had uncovered using the Share of Points % formula. That certainly held up during the Rugby Championship in which they escaped with a narrow win over the Springboks in Johannesburg before they beaten by the Wallabies in Sydney.

But looking ahead to the World Cup, Darwin said the ideas of cohesion and shared understanding weren't as relevant - particularly with the reforming of the Ma'a Nonu-Conrad Smith centre pairing; but he was more concerned for the defending champions because of their lack of a "Plan B" and the distinct dearth of an obvious back-up playmaker.

"The biggest concern for New Zealand is not cohesion; it is injury because they really haven't come up with a 'Plan B'," Darwin told ESPN. "They're starting XV is extraordinarily good; at the end of the last World Cup my opinion was that they should have dumped Dan Carter and Richie McCaw, which is an horrific thing to say, but to say 'actually let's rebuild here and put something together'. Whereas what they've had instead is [Aaron] Cruden, Carter, [Beauden] Barrett; in, out, in, out; they've not really been able to be totally comfortable with what's there as a back-up.

© Anthony Au-Yeung/Getty Images

"When they're starting XV backline is on-song, it's just extraordinary and it's better than anyone else in the world. But what they don't have is a second set of options; so they've just got to get through the pool stages without injury and then just hope to God they can get through it because if they do they've just got the best team in the world. But they just don't have a Plan B and with Carter's injury record that's the biggest concern."

The back-up situation is largely born out of Cruden's injury during this year's Super Rugby season; the Chiefs fly-half forced to undergo season-ending knee surgery after going down early in the tournament. Cruden had become arguably the All Blacks' first-choice No.10, albeit with questions around his goal-kicking, after the silky-skilled fly-half benefiting from Carter's own run with injury.

The All Blacks included Colin Slade and Beauden Barrett as Carter's back-ups for the tournament, both of whom have battled injuries of late themselves. Arguably the better option may have been to go for one-Test playmaker Lima Sopoaga, who starred in Johannesburg and who also enjoys a large degree of shared history with All Blacks No.9 Aaron Smith from the Highlanders.

If the All Blacks fans should be a little concerned with their frailties at No.10, things could not be any more rosy one position further out. The world champions have Nonu and Sonny Bill Williams to call on at second five-eighth, yet the decision for coach Steve Hansen, according to the Gain Line theory anyway, is clear.

"I think for us, what we can see is that New Zealand were a really, really good side between 2008 and 2011 when it was very heavily dominated by Canes and Crusaders," Darwin said. "And as they've started to go away from that they've become a little bit more inconsistent, particularly defensively. The second team that they put out in Auckland [against the Wallabies] was a little bit more leaning in that direction, particularly around the centre pairing. So while Sonny Bill Williams does give them a bit in attack, he gives away a lot in his understanding with Conrad Smith or whoever he's going to play with in that sort of midfield channel.

Ma'a Nonu© (Photo by Hannah Peters/Getty Images)

"It's extremely important that guys have an understanding with each other, and that's what Gain Line measure; we're not trying to measure skill level although we feel we're in a much better position to measure skill level. The great example of this is when you saw Ma'a Nonu bounce around between Super Rugby teams. He was nowhere near as effective when he was at the Highlanders, when he was at the Blues; and yet he was continuing to be effective for the All Blacks when he came back to working again with Conrad Smith. And that's because of what we call "shared history"; so the shared history between those two guys.

"And so they just understand each other defensively, the lines to run, little nuances; the things you don't have to say to each other on the field do have an impact. Now there's no question that people can work together when they do have individual skill sets; but [shared history] just helps you that little bit extra in getting people across the line."