This is why Donno is probably going to be on the bench though isn't it? He can cover 10 or 15 which are the main issues. edit +goal kicking
I think the backup 10 will be there as a 10 alone, fullback coverage won't really matter much - Sua'ali'i, Kellaway and Jorgensen all cover fullback and I'd be amazed if we named a 15 without one of them (likely 2).
Paisami is pretty likely the other bench back if we are going 5-3 - he covers 12 and 13 if either centre goes down and Sua'ali'i slides out to anywhere in the whole back 3 if something goes wrong there. It's the only way we can get full coverage of the backline as it stands.
If we go 6-2 than Donaldson does make more sense in terms of broader coverage from the bench.
5-3 or 6-2, the only way we have completed coverage is if Ikitau and Paisami are in the 23, at least from the squad as it stands. Given the modern realities of HIAs and the lions strength in the centres it would be a huge gamble to not have that channel properly covered.