Hansen has a knack of accurately gauging emerging talent so when he says 22-year-old Harris is ready, there's a weight of past evidence that says the coach should be believed.
One thing is for sure, Argentina will ask more than enough questions of Harris - who is expected to play at least 20 if not 30 minutes off the bench - for everyone to be better informed about the Chiefs hooker.
The Pumas will inevitably see Harris as a player to isolate and exploit in the scrums. Because they are smart and capable scrummagers, they will find a means to get their tight-head and hooker to be ever so slightly angled towards Harris and channel the pressure through his spine.
They will compete for All Black ball when Harris throws - why make it easy when he'll be nervous and eager to start well? And if they can smack him about a bit in the cleanout and on the floor ... all the better. Test rugby isn't supposed to be easy. Or fun for that matter. Not in the front-row at least.
None of this is news to Hansen. He's been around test football since 2001 and he knows exactly what Harris will be facing tomorrow and is backing his man to come through.
The challenge is tough, but so too is Harris, the All Black coaches believe.
And it's not as if Hansen has rushed Harris. A test cap is not something Hansen has been willing to give anyone, certainly not Harris, on a 'let's see how you go' basis.
The project to find the next test-capable hooker began more than a year ago. Andrew Hore signalled last month that he'd be pulling the plug on his career after last year's Rugby Championship.
With fellow veteran Keven Mealamu also uncertain about how much life was left in his legs, Hansen began his apprentice scheme.
Throughout the home leg of last year's Rugby Championship, an emerging hooker spent a week with the All Blacks.
The idea was that one of Harris, Rhys Marshall or Liam Coltman would prove they were ready to tour Europe in November. Hansen, having seen all three close up, decided that none of them were tracking well enough so Hore was persuaded to delay his retirement.
By June this year, Hansen still wasn't convinced he'd seen enough so ran with just Dane Coles and Mealamu for the series against England, with Corey Flynn on standby.
By the time the Rugby Championship kicked off, he'd at least fixed on the best choice of the emerging brigade. In Harris, the selectors saw a player with the mobility, dynamism and athleticism to play the type of game they were after.
At 1.85m and 106kg he has the frame to handle himself and will end up playing comfortably at about 110kg in time. Some big boxes ticked.
But being able to run about like a loose forward is only half the requirement for an All Black hooker.
What Coles has done this year is set exacting standards in regard to the core business of lineout and scrums. His throwing success sits at 86 per cent -- he's had 36 throws and missed with six.
There's still room for improvement but Coles has been a big part in transforming the All Black lineout into a genuine weapon.
He's also been a big part in a big scrum. Coles has done the basics particularly well while also posting competition-leading numbers for defenders beaten, passes and offloads.
Harris can't be expected to deliver those numbers on debut, but the very fact he's being given a debut is proof the coaches think that in time, he will be achieving those benchmarks.
- By Gregor Paul