Rugby: Beauden Barrett, Brodie Retallick battling to regain All Blacks starting spots - Sir John Kirwan

All Blacks icons Beauden Barrett and Brodie Retallick may be headed for the bench, once they return from their sojourns in the Japanese Top League later this year.

Both are serving sabbaticals, negotiated as part of NZ Rugby contract extensions through to the 2023 World Cup.

But while the former World Players of the Year are available for national selection later this year, All Blacks great Sir John Kirwan insists neither should walk straight back into the starting line-up on their return.

Sir John has told Sky Sport's Breakdown the pair will need time to adjust to the higher standard of international rugby, after missing Super Rugby Aotearoa.

"There's no way the All Blacks selectors would risk them coming back without knowing the intensity of the Japanese league," says Kirwan, who has played and coached in that competition. "Who is tackling out there?

"Maybe a game at the end of the season would sort that out, but Beaudy and Brodie have to come back and sit on the bench.

"You cannot let our guys play through that [Super Rugby] season, while they're away, and let them come back and make the team."

Barrett is currently the Top League's leading pointscorer, averaging 18 points a game through seven rounds, but his NZ selection chances are also compromised by the Super Rugby form of his closest rivals - Crusaders first-five Richie Mo'unga and Hurricanes fullback (and brother) Jordie Barrett.

Mo’unga has effectively replaced Beauden Barrett as first-choice No.10 for the All Blacks, while the younger Barrett can longer be denied the No.15 jersey.

"He's really taking command and owning that 15 jersey," former All Blacks fullback Mils Muliaina told Breakdown. 

"I think Beaudie's not getting the intensity at the moment. Jordie's getting that, he's putting the mileage in and I love the leadership role he’s taken on himself."

But Kirwan has even worse news for another All Blacks incumbent playing his trade in Japan - halfback TJ Perenara, who is currently off contract with NZ Rugby.   

"You cannot re-sign Perenara," he says. "I think there are some young guys coming through. 

"I'd love to sign him, but if it was a case of signing a couple of young guys to get us to the next World Cup and further... 

"He's been a great servant of the game, he's in the twilight of his career and there are guys that are starting to stack up that will give us another two World Cups."

While Aaron Smith and Brad Weber should retain their All Blacks spots on form, the selectors have lost another contender for Perenara's, with Highlanders half Folau Fakatava sidelined for the season with injury.

Join us at 7pm Friday for live updates of the Highlanders v Blues Super Rugby Aotearoa clash