Up until now, it has been an either or scenario for the All Blacks when it comes to Damian McKenzie and Richie Mo'unga.
They have taken the view that when Beauden Barrett starts, the bench can only accommodate one other first-five – hence they must choose between Mo'unga and McKenzie.
And so far this year, they have chosen McKenzie. There are two primary reasons for that.
The first is that this year, unlike last, the All Blacks coaches predominantly want to keep Barrett, who is also an accomplished fullback, in his preferred role at first-five for the 80 minutes.
They would rather not shift Barrett, yet they want a way to change the dynamic of the backline later in the game and they also need to have a genuine No 10 on the bench in case of injury.
As a result, it has mostly been McKenzie who has taken that bench role. Having started nine tests last year at fullback, he's not only got the specific positional experience he has the greater generic test experience and he also has the speed, agility and awareness to damage tired defences from the backfield.
It's a strategy that has worked well for the All Blacks this year and McKenzie has made the sort of impact that coaches crave and opposition coaches dread.
But the way things panned out in Buenos Aires last week have given cause for a re-think. McKenzie was unavailable due to a family bereavement and Mo'unga slipped on to the bench and then on to the field to produce a high impact 22-minute cameo at first-five.
While it may be the coaches' preference to keep Barrett at first-five for the entirety of each test, he showed against the Pumas that he can still be influential at fullback.
It was at fullback, after all, that Barrett first came to prominence. He used to come off the bench earlier in his career and slot in at fullback, albeit with a generous licence to pop up at first receiver.
That Barrett can still run a game and change its destiny from fullback wasn't the big news in Buenos Aries.
It was the way Mo'unga injected himself and turned the game which surprised.
He hasn't had much experience with either the Crusaders or All Blacks at coming off the bench and has, as a consequence, been considered more of a starting 10 than an impact 10.
He went some way to changing that perception last week. Mo'unga gave the All Blacks a new threat when he came on.
He attacked the space close to the ruck and invariably slipped into a hole or found a way to get someone else into space through the midfield.
He also created a try for Anton Lienert-Brown with a deft kick and while it was only one test, it was enough, presumably, to make the selectors wonder whether they could potentially pick both Mo'unga and McKenzie on the bench for the clash with the Boks in Pretoria.
It would be unusual to have three first-fives in a matchday 23 and as much as it would come with some risk, it would also come with significant potential reward.
If the All Blacks want to try to turn the final quarter into an unstructured aerobic ordeal for the Boks, they might fancy having all three of Mo'unga, McKenzie and Barrett on the field to do that.
Mo'unga could run things at 10, Barrett could be at fullback and McKenzie on the wing. Rieko Ioane could shift into centre and Ben Smith, regardless of where he starts, could be on the other wing and with those players on the park, the All Blacks would have a merciless capacity to attack at a time when fatigue is biting and space opening up.
It would also give them more kicking options to turn the Boks if need be or to contest more for the ball in the air and with three first-fives on the field, surely, if it came to it, one of them would be able to drop a goal?