Reasons why you might want to bring him on: the lineout is creaking, too much possession is being lost in the backline, hitting in the tight isn't working, he's a strong and fast carrier to bring on, the backline are struggling defensively etc. In fact, as a try-scoring forward who'll provide more in the loose, perhaps it could be argued that he's a perfect game changer.
And there are plenty of ways to accommodate Croft and SOB. Since Warburton isn't likely to come off, either Croft goes into the second row and SOB replaces Lydiate, or Croft comes in at 6 and SOB replaces Heaslip.
Hmm, lets examine that reasoning.
Lineout getting it tight - Croft is not a 2nd row option then, could use him in the backrow, but is he much addition over Heaslip or Lydiate at the front of the lineout?
Lost possession in the backs - You want a ball carrying forward? SOB
Going tight is not working - You need a better carrier in the backrow? SOB
Strong carrier? Nope. SOB
Fast carrier? Yep, but faster than Davies, North, Bowe, 1/2p, nah, you'd be safer working on getting them the ball in more space.
Backline (as opposed to backrow) struggling defensively? Did you not notice Croft was one of the chief men at fault for the two tries Australia scored at the weekend, Australia exploited his lack of knowing how to defend the 3/4 or wider channels.
So, in summary, of the two reasons he could be required:
-Croft is a marginally better option at the front of the lineout than any of the other backrowers... but the improvement is marginal at best.
-Croft is quicker than the other forwards - but you need to get the ball out to him in space to use his speed as he isn't good in heavy traffic - and you need to get him the ball in wide channels quickly with one less man clearing rucks. Catch22.
