The Crusaders prop, capped 55 times by the All Blacks, has returned to the operating table for surgery on an eye injury sustained playing for Canterbury against Bay of Plenty last weekend.
The 29-year-old returned to rugby earlier this year after 10 months out after two achilles operations.
New Zealand forwards coach Steve Hansen said: "The surgery he is having is under a local and I don't think it's major.
"He is still a good chance of going to the World Cup as long as the surgery goes well and he recovers quickly.
"I saw him go down and being tended to but he continued to play on so I brushed it off and thought he'd got away with it."
"We were very happy with how he was going, particularly with his scrummaging which was the major concern. We're very happy with the Achilles and he's probably fitter now than he's ever been."
The All Blacks have left a place open in their World Cup squad for Somerville, but the 30 players have to be confirmed by August 14.
Any player who is replaced after that date cannot be brought back into the squad during the tournament.
Hansen added: "Nothing's really changed. We've just got a focus now on an eye injury rather than an Achilles."
Wellington's John Schwalger replaced Somerville during the Tri-Nations, but selectors could also look to Auckland duo Saimone Taumeupeau and John Afoa as alternatives should Somerville fail to recover.
Carl Hayman, Tony Woodcock and Neemia Tialata are the other props selected.