And no one in the 23 has less experience than Halai who began this year's campaign with the Blues having played only 25 senior games. But Halai has been too hard to resist: he has belied his inexperience all year, making a mockery of those who said he wasn't equipped to make it even to Super rugby level.
The 25-year-old was the man Super rugby didn't want between 2010 and 2012. He was ripping up the World Sevens circuit but in the 15-a-side game the prevailing view was that he was defensively vulnerable.
Kirwan was prepared to take the risk and reaped immediate reward.
There has been little sign of frailty, with Halai expertly shutting out Julian Savea earlier in the season and barely missing a tackle or being caught out of position.
He's also been solid under the high ball and at 1.95m and 110kg, he is the sort of athlete who makes an impact when he has the ball.
Given Japan's status in the world game, the expectation is that Halai will see plenty of action, but coach Steve Hansen was eager to preach caution on that front.
"The three selectors have been very mindful of using this game to gather some more information on our younger players but also using it to provide [Richie] McCaw and [Daniel] Carter with some game time.
"We are expecting the Japanese to be very competitive and it will be important that we respect them and engage them in the physical aspects of the game before attempting to play too much 'razzle dazzle'-type rugby ... If we don't do this we can get caught up in the type of game that suits them."
McCaw's appearance in the No 8 shirt is partly about being aware of the need to have adequate cover for Kieran Read in that role should it be needed later in the trip and also partly about giving Sam Cane more time in his specialist role at No 7.
Cane has emerged this year as a genuine force at test level and provides some comfort that McCaw can be replaced if needs be. But it's a different story with Read.
Luke Whitelock has been taken on this tour to be developed as a long-term option at No 8 but in the short-term, McCaw is the preferred back-up.
All Blacks v Japan
Tokyo, 6pm Saturday
15 Beauden Barrett, 14 Frank Halai, 13 Ben Smith, 12 Francis Saili, 11 Charles Piutau, 10 Daniel Carter, 9 Tawera Kerr-Barlow, 8 Richie McCaw, 7 Sam Cane, 6 Steven Luatua, 5 Dominic Bird, 4 Jeremy Thrush, 3 Ben Franks, 2 Dane Coles, 1 Wyatt Crockett
Reserves:
16 Andrew Hore, 17 Jeffery Toomaga-Allen, 18 Charlie Faumuina, 19 Brodie Retallick, 20 Luke Whitelock, 21 Aaron Smith, 22 Tom Taylor, 23 Ryan Crotty.
