Oh my...could be shock, or internal bleeding. Even a heart attack from just the fear.
Maybe it'll recover...
I always try to give them a chance, but these little injured critters never make it. I didn't inspect him completely either, didn't want to stress him. I figure, better to have some peace and quiet to die in than getting ripped up by a cat.
I put some horse grain out for the others, they seem to be sniffing it trying to figure out if it's food.