The All Blacks have suffered an injury blow a week out from the start of the Rugby World Cup, with rookie flanker Luke Jacobson forced out of the tournament.

Jacobson, 22, was diagnosed with concussion and will return home for treatment, with his place in the squad taken by loose forward Shannon Frizell.

All Blacks coach Steve Hansen felt for Jacobson, who suffered concussion symptoms last month after his Test debut against Argentina.

He was deemed ready to face Tonga in last Saturday’s 92-7 win in Hamilton, when he came off the bench, and didn’t present with any concerns afterwards.

However, he developed symptoms after a training session in Japan this week and All Blacks medical staff diagnosed a delayed onset of concussion.

“We all felt that the best thing for him to do is to take a break from rugby until he is symptom-free and ready to return to rugby,” Hansen said.

“While it was a tough conversation to have, ultimately it was a very simple decision. We have a duty of care to look after our players and Luke’s health is the most important thing above anything else.”

Former age-group star Jacobson has been labelled a future All Blacks captain by some pundits, but he was unlikely to win a place in the strongest XV for the big matches at this World Cup.

“I’m gutted to be missing the Rugby World Cup,” he said in a statement.

“However, I completely understand that this decision has been made with my health and best interests at heart. Whilst I’m feeling 98 per cent okay, I know I’m still not quite right and by taking a break now, I’ll give myself the opportunity to continue my career in the future.”

Athletic ball-carrier Frizell is the brother of St George Illawarra and NSW rugby league star Tyson Frizell.

Following his debut last year he started all five of his Tests, including last month’s draw with South Africa.

The 25-year-old has potentially benefited from the ill-fortune of others.

A New Zealand Rugby statement said two capped loose forwards – Vaea Fifita and Dalton Papalii – weren’t considered for selection because they are nursing injuries.