This is a good question. Is he a borderline or solid RB3/Flex each week here on out? One concern is that Bush's numbers drop if/when Thomas comes back. Not sure if I'd have the confidence starting Bush regularly, even as a flex, unless the matchup is great. If Thomas comes back, Bush's presence will benefit Thomas more than himself.
I like Bush a lot, but don't think he will be anything more than an RB3 going forward with inconsistency be the key. I would lump him in with other RB3-like players such as Thomas, Forte, BJGE, Jacobs, M Bush, Addai, Thomas Jones, Moreno, etc. Let me clarify that many of these RBs I mentioned are RB2 talent-wise, they just haven't been scoring consistently enough for me to confidently play them as my RB2 every week.