Exactly what would he learn in the role? And if he's coming with the intention of being elevated to head coach in 2017, why not just start there? Why not just make him head coach now as, honestly, what's in it for Umaga, the players and the club waiting one more season for a handover of power?
The Blues board - or at least some of them - have been taken with the idea of building a sustainable coaching structure where there is limited, if any, need to go to the market for a head coach.
They talk of a pyramid, where the head coach works with an identified assistant who is groomed to succeed when the time is right. In theory, it's an idea that has merit.
In practice, and specially if put into practice next year, it will be riddled with flaws. The first part that has to be challenged is why retain Sir John Kirwan at the apex of the pyramid?
The basis of the sustainable model is to ensure the protection of institutional knowledge and coaching capital. Continuity - as demonstrated by the All Blacks - can lead to seamless coaching transitions.
But the Blues dismal record over the last three seasons gives reason to question how much coaching capital and institutional knowledge Kirwan possesses. It's a stretch to suggest the Blues would fall apart with him or that he holds the key to the franchise's hoped-for brighter future.
If Dave Rennie were to walk out of the Chiefs, it would be damaging. If Jamie Joseph were to leave the Highlanders, it would be a problem, as it would be if Todd Blackadder were to move on from the Crusaders. These men - and give Chris Boyd another season at the Hurricanes - have in-depth, high-value knowledge of their team and respective set-ups.
Kirwan will argue he does, too, but a 33 per cent winning record says differently and the board absolutely have to ask whether the club would be damaged by letting him go now. They have to ask whether their sustainability model needs Kirwan at the helm in 2016 or whether they'd be better to ask Umaga to be head coach and pick and groom his own successor.
All of this could be rendered moot anyway as Umaga has been asked before to be part of the Blues and didn't fancy it last year.
He's quite content with ITM Cup and the balance that brings to his life. He knows that, if he has genuine ambition to progress, he'll have to take the plunge into Super Rugby at some stage.
Whether that's now, as Kirwan's assistant with a wink and a nod that he'll be head coach by 2017, is something Umaga will be in no particular rush to determine.
