The article linked in the first post in this thread makes the case that between Mack and Donald the first to sign will reset the market for non-QBs and then the other guy will up the ante. It makes sense in light of that that Donald would want Mack to sign first. And if that's the plan it makes total sense that Donald avoid injury until Mack signs.
OBO got hurt in OTAs. If that was Donald he'd be screwed. It's entirely possible that Donald's holdout has more to do with staying healthy until Mack signs that it does with Donald's displeasure with the Rams offer.
AlbaNY_Ram