I'm not sure that Gay is injured too badly, although there is talk of his 100 metre bid in doubt. That said this could all be spin as it appears it was only severe cramp of the left hamstring that cost him a place in the 200 metres due to the harsh US trial rules. A psychological blow as much as anything as he cannot repeat his 100/200m double of the World Championship..
One person who certainly can attain the Olympic double is
Usain 'Lightning' Bolt He is the 200 metre specialist, but the way he has been tearing up the 100 metres has been impressive. To look at him you would think he isnt quite the 100 metre world record holder. Tall and bulky, but his power is explosive... He still hasnt fully committed to the 100 metres, but surely there is no better incentive then to do the double with Gay out of the 200 metres..?
Ok, Gay did 9.68

, although it was wind assisted, but still... You couldnt split them based on that, but with the 'injury' and the fact that the Bolt looked mightyly impressive in the Jamiacan trials (the the way to slowed down around 20 metres to go, outrunning Poweell and still clocked 9.85 was vey impressive..)
The consensus is with some of the bookies that the 'injury' was the blip and with the 9.68 time Gay is the fave, but the Bolt is looking good as well and 7/4 in what I think is a 2-horse race (Powell won't catch them up, no matter what he says about building up gradually for the games,) is a nice price still, as you just don't know what really is the njury with Gay...
The Lightning Bolt to win the 100m Olympic Gold - 20 pt stake @ 7/4 (Corals)