It's not about how good they are. Staines has been in the 21 and also picked before MWZ until Mandatory isolation and lockjaw happened. Ivan has made the pecking order very clear.
MWZ is a pretty dependable back up. If he going to figure in our fu
Staines definitely comes straight in if he is fit, his debut has more than earned him another shot with To'o being out.
Who would Staines be marking up against? That may have some bearing on Ivans decision making?
Personally, I think Ivan will play it safe and go with MWZ. Just my opinion.