She was definitely attacked by a predator, maybe a dog. I'm glad you took her back, put please do not bring her back to the park again after this if you manage to get her healthy once more. Domestic ducks do not do well at parks, as you have seen. If you can't keep her, please find someone who can and give her to them.
Now, onto the wound. Firstly, since it was covered with flies when you found her, you're going to need to keep a sharp eye on it for flystrike - maggots in the wound. In fact, make sure you check her good all over under all her feathers for other wounds too and check them for flystrike as well.
Clean the wound off with water and apply an Espom salt compress. This will help with any swelling and will help draw out any infection if there is any.
After that, apply neosporin to the wound. Repeat this process daily.
You may also want to let her swim in a warm tub of water each day to exercise her wounded leg. If we're lucky, it's just soft tissue damage and will heal over time. Worst case scenario, it's a break or, due to the position of the wound and where it looks like she was attacked, spinal damage.
Does the leg feel hot to the touch?
Really the best thing to do here if at all possible is to get her to a vet and have x-rays done to see what's really wrong with the leg.