Don’t just let your duck limp around because it may get worse with time. Catching a duck can be tough, so do your best to catch the injured duck in a calm way that doesn’t stress the bird too much.
This, of course, is always a good course of action anytime you’re concerned about an animal. You can take the bird to the vet, or take some pictures and video of the injury/limp and send it to the vet.