In just four months Smith has become a vital part of McCaw's team.
In terms of rookie season impact, Aaron Smith can lay claim to being the most significant All Black find since Christian Cullen.
It takes a special talent to be immediately dangerous in test football. Only a handful ever manage it - make an almost obscenely quick transition from debutant to integral component.
Smith is in this treasured group. His ascent has been incredible: he began the season as a feisty and admirable back-up to Jimmy Cowan at the Highlanders and tomorrow he will win his 10th cap. More significantly, it's already scary to imagine life without him.
In the absence of the injured Will Genia, Smith may even be the pre-eminent halfback in world rugby. The claim is not outlandish - only the bruising Mike Phillips of Wales looks to be a serious challenger - but the speed of the achievement is.
The scale of Smith's offering is such that the All Blacks simply wouldn't be the team they are without him.
Piri Weepu can't deliver in all the areas Smith can. Weepu, in truth, can barely tick half the boxes of his younger rival and his days of making a genuine impact are confined now to games that remain locked in bruising, grinding conflict; games that haven't shaken loose and are still in need of a tactical navigator late in the piece to nudge and probe at a more leisurely pace.
It's mostly of little comfort these days to see Weepu enter the fray - what it signals is that the All Blacks won't be able to play with the width and pace they can when Smith is on.
Blink and miss things when Smith is about: the ball sits at the back of a ruck for all of a millisecond before he whips it away. He's omnipresent, too, seemingly magnetic in the way he's drawn to every breakdown.
His passing has range and empathy while his wider game evolved in the Rugby Championship - his running, kicking and decision-making all looking to be top-drawer assets.
Imagine the frustration that would have brewed had Weepu started in either or both of La Plata and Soweto. It would have been intolerable after encountering a dry ball and fast track in both venues to then see Weepu crouched over the ball, gesticulating wildly to his forwards to retreat. Whole minutes of the game lost to this pantomime of nothing.
There is none of that with Smith and his contribution in Brisbane is likely to once again be vital. The All Blacks can't surely have failed to spot the lack of grunt on the Wallaby bench and will have it in mind to run the Australians off their feet knowing there is not much of a cavalry poised to enter the fray.
In taking just four months to become such a valued All Black, Smith's journey is more rapid than that of even Dan Carter or Richie McCaw. Carter was first capped in 2003 but took until November 2004, when he was shifted to first-five, to really shine. McCaw was always impressive but he continued rather than immediately enhanced a succession of quality opensides. Even Jonah Lomu had one ugly season to get out of the way before he settled.
Cullen scored seven tries in his first two tests in 1996 and had an iron-grip on the All Black No 15 before the Tri Nations even started that year. He pushed on to become an undisputed great - something Smith will surely do, too, if he can sustain what he has started.