The Admiral picked up a knee injury in the game against Bolton at the weekend when he landed awkwardly.
Scans have revealed that he has torn a muscle behind the knee and suffered extensive bone bruising.
This means the 32-year-old will not be back in action for around six weeks, but it does mean that the player will be available for New Zealand’s World Cup campaign.
"It's probably the best result I could have got," Nelsen said.
"In the great scheme of things it's not too bad. It certainly could have been worse."