Well right now, one thing you can do for sure is to watch if its still able to eat, drink, and poop. It sounds simple, but this is one of the things people will watch with sick animals.
You don't have to have it eat right away, but just see if it will still be able to eat in the next few hours.
If you isolate the duckling, you could try to bring it in the house and put it in a box to nurse it back to health. But because their minds are herd animals where the social network and herd is safety that could stress it out even more if there's not a second (or third?) duckling with it, to help keep it from panicking even more. But bringing it into the house to keep a closer eye and better watch over it may give you some recovery advantages.
On top of watching if its eating, drinking, and pooping you will be able to watch it closer inside to determine if there's internal bleeding, such as with blood in its stool, etc. And some blood might not be a game over, it may depend on how much there is.
It wouldn't hurt also to do hand feedings for a bit, or various means of helping it eat and drink also to reduce travel stress on wounded body parts for a while.
