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Former All Blacks lock and Crusaders coach Todd Blackadder believes Williams' spot in the team will be under threat following unconvincing performances on the northern tour.
Williams (33) yesterday hinted that he may continue playing after next year's Rugby World Cup though he could hardly put a run of games together due to an injury-ravaged season.
A shoulder injury suffered against England ruled out Williams against Ireland and ended a frustrating year for the midfielder. Though he's still the incumbent second-five, Williams' form and fitness are becoming increasingly worrying.
"Sonny Bill Williams' place in the team will be one of the talking points; there'll be discussions by the coaches around that and combinations moving forward," Blackadder told The Rugby Paper.
Now the coach of English club Bath, Blackadder has been one of Williams' most vocal supporters, having coached him at the Crusaders in 2011.
He previously described Williams as "critical" to New Zealand rugby and compared him to All Blacks greats Richie McCaw and Dan Carter.
Though Blackadder insists the Kiwis' struggles against England and defeat to Ireland won't hamper their chances of an historic World Cup three-peat, he admits being concerned over the team's inability to deal with the defensive tactics employed against them at Twickenham and Lansdowne Road.
"I actually think this is just the start and that the All Blacks have been given a real gift by being tested so bloody hard during this November series," Blackadder said.
"Both England and Ireland brought a lot of line-speed and defence is what won the game for Ireland on the day, and might have done for England as well.
"When the All Blacks have lost it's been on the back of a massive defensive effort by the opposition, so they'll look closely at their attack, how they can keep teams under pressure and look after the ball better, because they've had opportunities they didn't take."
Ireland will be the All Blacks' biggest - but by no means only - challenger in Japan, Blackadder said.
"England can also take heart from the fact that while they probably haven't got their best team available at the moment, they still pushed New Zealand close and could well have won.
"Wales are looking very strong and Scotland are improving, so it should be a hell of a Six Nations and I'm looking forward to watching that. But I still think where the All Blacks have the edge is they're a little bit more consistent and that could tell in Japan."