It's all about the World Cup.

That is the obvious inference to take from All Blacks assistant coach Ian Foster's comments, days out from the Rugby Championship clash against the Pumas in Buenos Aires.

The All Blacks have left out key players including the great locking combo of Brodie Retallick and Sam Whitelock for the Argentina test on Sunday morning, saving them for the Springboks.

Foster said: "The challenge for us is an exciting one and if you dwell on who is not here, you forget who is here.

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"It is a chance to build real competition and depth in the squad, which is vital for where we want to get to over the next couple of years.

"It might feel uncomfortable when some of your so called number one players are not on the field. Long term, it is a massive opportunity to grow the number of international players."

Foster admitted Argentina had "surprised us in a couple of areas" in New Plymouth earlier this month, and said there were parts of their game the All Blacks were injecting into their own schemes.

"We've talked a lot about the New Plymouth game," said Foster, who also praised Pumas coach Daniel Hourcade.

"In the last five or six years they have become a real force in world rugby. Daniel has done an amazing job.

"If you look at the offloads and defenders beaten...they are high in those areas which is not a traditional part of the Argentinian game.

"In some ways they have taken a real gamble but I think they have reaped the rewards. Look at the World Cup - they played outstanding rugby.

"With it comes risks at times when it doesn't come off. We've been there this year. The hard part is getting the balance right."

Meanwhile Foster refused to give any hints when the All Blacks would introduce a rising halfback to replace Tawera Kerr-Barlow, who heads to a French club after this season.