Many disabled birds can be difficult to care for, however ducks are among the easiest since they seem able to handle adversity better than most animals, including other birds. Of course, knowing how to care for the duck is most of the battle.
Two very important things to note right from the beginning are that the term 'disabled' is so broad it can mean nearly any infirmity or illness, and that in many areas, it is unlawful to care for a wild disabled bird or animal without special credentials, generally as either a vet or wildlife rehabilitator. If the duck in question is from the wild, be sure to check local laws before endeavoring to care for the animal, except for temporary care until you can get the bird to someone with the credentials that allow them to give the required care.
How to and the amount of care the duck is going to require will have a great deal to do with what is wrong with the bird. The duck may be missing flight feathers, have a broken wing, have a broken leg, have shotgun pellets embedded, may be coated with oil, may have puncture wounds, may have various illnesses, and so on. Each of these would qualify as disabling the duck, but some are far easier to deal with than others.
Missing feathers are easily regrown it just takes time. That just means giving the duck plenty of good food, shelter, fresh drinking water, and a place to swim until the feathers have grown back.
Broken wings or legs usually require splinting and wrapping. Done properly, the duck will recover well, if they are kept warm and their other needs are seen to.
Chronic illnesses are even more difficult. We had four moscovie ducks that all had chronic problems with their leg joints. For those who don't know the breed, Moscovies are huge ducks that look like a three way cross between a duck, a goose, and a turkey. When I say huge, I mean it. Both drakes weighed over 25 pounds, and the hens were 'only' 20 pounds.
Since the bone structure is heavier for moscovies than with most other ducks (and many geese as well), that much weight on the joints would cause swelling and pain. Treatment was actually pretty easy, though it did take a little thought. We got a large children's swimming pool they could get into easily, and filled it with water. The ducks would spend hours on end, just swimming. They'd get out occasionally to dry off, but it wouldn't be long before they were back in the water. Since the water buoyed them, there was no weight on the joints, so pain and swelling went down. The hardest part was in keeping the pool thawed out during the winter.
A vet should treat illnesses. However, rehabilitators often work closely with vets, and many have some training in that field of animal medicine.
The biggest challenge we faced also gives a great idea of how resilient ducks are. We were brought a wild duck that was missing a leg. That part had to be treated, wrapped, and healed, and the duck would never again be able to be wild because of the injury, however a year later, most people were surprised to find out about its disability. It got along fine, and in the water, there wasn't a way a person could tell it only had one lower leg.
This was an extreme example, but it does show how hardy and tough ducks are. Injuries they have must be taken care of properly however, with the right care, ducks will find ways around most disabilities. In fact, people could learn a thing or two from them. With proper care, a disabled duck can recover better than most disabled people.
It should also remembered that ducks require shelter, food, drinking water, and water for swimming. The food is important as good food promotes health. They should be fed grain daily, especially in the wintertime, fresh greens when possible, and it should be noted that most will eat small aquatic insects, little fish, and earthworms. They also love occasional soft fruit. (Balance the dry with the green foods, though. Too much of either may cause problems.)
Drinking water should be kept fresh, though duck will often put greens in the water and dabble for them. This is normal and is no reason for concern as long as the greens are removed before they start to rot.
Some care should be given to keep the disabled duck away from more aggressive animals, especially dogs. While the birds will often do a good job of fending for themselves, they are still disabled in one fashion or another, so they don't have the full range of defenses they'd normally have. Sometimes they will still go begging for trouble, though, so keep an eye on them.
Caring for a disabled duck is quite a bit easier than caring for most disabled animals, outside of basic care and within consideration of what the disability is. Most ducks, even those that are quite wild, also domesticate easily. So the care isn't hard and it does have rewards. Still, it is a good idea to have a plan in place before you get too deep in the project. If you do, your chances of success are great.