The trick to lifting up a buddy who’s been injured is to grab him from the middle. If you manage to divide his weight equally on either side of your shoulder, you should be able to schlep him a good distance without feeling much pain.
The downside is – he’s probably going to feel a lot of pain. In basic training, where I learned the art of schlepping a wounded friend, we saw that an average-weight man is able to carry a man much larger than himself using this technique – but that other guy will be living with the agony of a bony shoulder in his gut – never mind the theoretical injury he was supposed to endure.
So, in conclusion, don’t carry an injured man on your shoulder unless it’s absolutely necessary. And pace yourself – it’s the quick walk that pushes the shoulder into the gut.