Former All Blacks coach Laurie Mains has questioned whether Sonny Bill Williams is past his use-by date as an All Blacks midfielder.

Williams hasn't hit his best form since being sent off against the British and Irish Lions, leaving questions about where he falls in the All Blacks midfield pecking order.

Williams, 32, is contracted with New Zealand Rugby until 2019, and has been given a start in the 12 jersey against Argentina tomorrow morning.

Mains, who coached the All Blacks from 1992 to 1995, told Mark Watson on Radio Sport that he believes Williams may be past his best in the black jersey.

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"I think maybe he's getting to the end of it. There's too many mistakes now, too many things that aren't working for him that worked three or four years ago," said Mains.

"Maybe he's achieved what he's going to achieve in rugby, that's what I'm feeling with him. He's got quite a long way to go to be what you'd call your typical All Black midfield back - who generally tend to be rock solid and don't make mistakes."

The selectors have given Williams another chance to impress in Buenos Aires, as well as Damian McKenzie, with the electric Chiefs fullback putting in mixed displays in his six tests to date. Mains argues that McKenzie needs to find the balance to make sure his attacking intent doesn't become reckless.

"While [the All Blacks] play to a pattern - that pattern gives every player a chance to express themselves, and that's not an easy thing to get in rugby. They wouldn't want to curtail him as much as educate him and coach him to read the situation a bit more accurately.

"He is making the odd mistake because he's trying to inject himself into the play too much. Whether he can become the top All Black fullback, I think the selectors are still finding that out. I find it interesting that David Havili, who is a bigger, stronger player, is now starting to hover around that area."

Selection pressure could also be applied to starting prop Kane Hames, who struggled in his first test start against South Africa.

"I didn't think he scrummed all that well," says Mains.

"He's got a bit of a tough job ahead of him to prove worthy of being there as a long-term All Black prop. If he does a good job at scrum time against Argentina, he should be able to do it against anybody - they're very strong front-row players."

Hames is one of several players given an opportunity in the starting lineup due to injuries and rotation, which Mains asserts will be a difficult task.

"All of those players who are coming into the team to replace world greats - what a huge responsibility, so a pass mark will be that team performing well and getting a comfortable win - by eight or more points.

"[Argentina] will be boosted by the fact that many of the top All Blacks aren't there - they will give themselves a chance if they start well, and if they do start well, look out, it'll be one hell of a fight."