The mighty All Blacks have seemed almost invincible in recent years but back-to-back defeats have left their new coach and captain under fire and raised an unthinkable prospect: losing three in a row for the first time this century.

Saturday's Tri Nations game against Argentina, New Zealand's final outing this year, is a chance to stop the rot after shock losses to Australia and Los Pumas in consecutive weeks.

But fans fear the three-time world champions have lost their way under coach Ian Foster and his captain Sam Cane, after poor performances left them looking rattled both on and off the field.

New Zealand coach Ian Foster has come under pressure from fans
New Zealand coach Ian Foster has come under pressure from fans AFP / Patrick HAMILTON

Before this month, New Zealand, one of the most successful teams in any sport, hadn't lost two in a row in nine years. The last time they lost three straight was 1998.

"Will Ian Foster be sacked if the All Blacks lose again?" New Zealand's largest news website asked this week, just five games into the national coach's tenure.

It is the nature of the losses, rather than the scoreline, that has All Blacks supporters fretting, with the New Zealand Herald labelling the performance against Los Pumas "shambolic, unimaginative, uninspiring".

The harsh criticism from New Zealand's often adoring press and fans has hit a nerve, with Cane, perhaps unwisely, lashing out at "brutal" comments from supporters and accusing them of not understanding rugby.

New Zealand play Argentina in Newcastle on Saturday
New Zealand play Argentina in Newcastle on Saturday AFP / David Gray

"You've just got to remind yourself that they may like to think they know a lot about the game of rugby but in reality they don't," he said, in remarks that put many disaffected supporters even further offside.

Despite Cane's defiance, the New Zealanders have looked vulnerable to rush defence in recent outings, responding with poor discipline when opponents get in their faces, rather than formulating a Plan B.

Foster admitted as much in a radio interview last weekend, saying he was as frustrated as anyone by his team's lack of tactical nous.

"Under a bit of pressure we got a little blinkered and tried to blast our way through with ball-carrying -- it didn't work against an outstanding Argentinian defence," he told Newstalk ZB.

Beauden Barrett said the All Blacks need to play smarter
Beauden Barrett said the All Blacks need to play smarter AFP / Patrick HAMILTON

One issue facing Foster is that many Kiwis see him as part of the problem, questioning whether he should have been promoted to succeed Steve Hansen in the top job after seven years as an assistant.

Critics claim the All Blacks, thrashed by England in last year's World Cup semi-finals, had become stale under Hansen and needed fresh ideas, pointing to Scott Robertson, the break-dancing, left-field coach of the Canterbury Crusaders.

Argentina beat New Zealand 25-15 in Sydney
Argentina beat New Zealand 25-15 in Sydney AFP / David Gray

Robertson had three Super Rugby titles from three attempts when he applied for the All Blacks' job last year and has since burnished his credentials further after the Crusaders won Super Rugby Aotearoa.

In comparison, Foster had an uninspiring eight-year spell in charge of the Waikato Chiefs, when their best result was a losing appearance in the 2009 final.

The contrast may be unfair, given the huge difference between Super Rugby and the international game, but it has become starker with every misstep the All Blacks have made during Foster's brief tenure.

Cane has faced similar questions, after he was a surprise pick over Sam Whitelock for the skipper's armband. He said Foster was withstanding the criticism, but added "he's only human".

"I've been really impressed with Foz, the way he's stood up," Cane said.

"He's given us really clear focuses on where we need to get better. It doesn't feel like we're bogged down and don't have the answers."

Playmaker Beauden Barrett said the All Blacks needed to display more discipline when faced with smothering defensive tactics, instead of gifting opponents decisive points.

"It's like a skill or physical training. We need to train the brain as well," he said.

Foster said the answer was adding variety to the All Blacks' attack, with more driving mauls and better tactical kicking, rather than simply running the ball at every opportunity.

"It's part of our DNA that we want to attack, you can't change that, but clearly we've got to get smarter in how we do that," he said.

Foster is arguably in a no-win situation as the All Blacks prepare to face Argentina in Newcastle on Saturday.

A convincing win would quieten, but not completely silence, the doubters who have always opposed his appointment.

A third successive defeat -- for the first time since John Hart's class of 1998 lost five in a row, three to Australia and two to South Africa -- and the calls for his head would become deafening.