New Zealand back-row forward Luke Jacobson has been ruled out of the World Cup because of concussion.

Jacobson played against Tonga last week and travelled with the squad for the tournament in Japan, but the All Blacks said the 22-year-old had suffered a "delayed onset of concussion".

New Zealand Coach Steve Hansen said the player showed symptoms after their first training session this week.

Jacobson, who has two caps, will be replaced by Shannon Frizell.

"After discussing this latest episode with Luke, as well as our medical team, the decision has been made to, sadly, withdraw him from the tournament," Hansen told the All Blacks website.

"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."