Why else are they hoping and taking the risk that DeCastro might be able to play before the end of the season versus getting him healthy on IR? Because they know they need better than Foster playing in games that matter. That has nothing to do with DeCastro being the #1 pick because the safe bet would be to put him on IR.
I guess the proof in the pudding will be if they retain Foster at the end of the season. My bet is they don't and that is not what they do for people they want to be starters.