I totally get that, what I'm saying is it's way easier said than done...I don't see it as quite that simple.
It's fair to say if Tua is out for the season or long term that, yes, it really doesn't make much difference.
Where I differ is that there is another scenario where Tua misses a game or three. The difference between winning the division and missing the playoffs might come down to a game or two.
The backup needs to be able to function efficiently and effectively within the scheme. He doesn't need to be a hero. He just needs to not be atrocious and not give away a winnable game.
Do we have that? Hard to say, but we have what we have. It isn't likely going to change at this late date.
First you need to ask yourself, who really has that and knew it before hand?
Then ask yourself who ever thought they had that and they were totally wrong?
It's basically luck.