All Black blow as Jacobson ruled out of World Cup

Tokyo - The All Blacks' Rugby World Cup campaign suffered its first setback on Friday when rising star Luke Jacobson was ruled out of the tournament due to a delayed bout of concussion.

The defending champions, who are chasing a third straight trophy in Japan, called up rookie Shannon Frizell to replace Jacobson in the loose forwards.

Coach Steve Hansen said the 22-year-old flanker showed no sign of concussion after New Zealand's 92-7 win over Tonga in Auckland last weekend but developed symptoms during the team's first training session in Japan.

"We have a duty of care to look after our players and Luke's health is the most important thing above anything else," Hansen said.

"Luke's a young man with a hugely promising career ahead of him and I'm sure he'll get the chance to play at other Rugby World Cups in the future."

Jacobson has played only two Tests but impressed enormously for both New Zealand and Chiefs, with some pundits even tipping him as a future All Blacks' captain.

"I'm gutted to be missing the Rugby World Cup," he said.

"However, I completely understand that this decision has been made with my health and best interests at heart."

Frizell, 25, is also a novice at international level, with five Test caps, the most recent a draw against South Africa in Wellington in July.

Hansen said two other potential replacements, Vaea Fifita and Dalton Papalii, were not considered for selection because they were currently injured.

The All Blacks arrived in Japan this week ahead of their first game against South Africa on September 21.

They will also play pool matches against Italy, Canada and Namibia in their quest for a fourth World Cup after victories in 1987, 2011 and 2015.