I have been in a 14 man .5 ppr league for a few years now so hopefully I can help.
I agree that starting Garcon is a huge risk with Manning out. I also think that starting Thomas on Monday night would be to heart-wrenching if you are down in points and Bush is catching a million passes and getting carries as well.
This leaves you with Starks or Knox. I personally don't like either back for GB but in a 16 man league you gotta do what you gotta do to get role players and the 47 point over under for that game Thursday night is promising I just can't trust the split back field.
I would personally go with Knox (maybe just because he used to burn people on Madden '10) but Atl is a high scoring team and will probably be passing more than ever. Chicago may be behind and throwing deep. Hopefully Knox can snag some passes and get to the endzone. I feel he has the best combination of good upside and less risk than the other 3 players. That's what I want in my flex spot.
Hope this helps. viewtopic.php?t=494672