We are in the midst of a 25 night cruise. As Mike said, the doctor and nurse are generalists but well trained in emergency medicine. I have no doubts that they could tend to a sprained or evern broken ankle. However, if it was anything "serious" or even something you might consider minor but they are unsure of; they will make you get off the ship.
As for rehab, your best bet is the fitness instructor. For a fee, they can set up a regime using equipment onboard. REMEMBER that they are fitness instructors and not physical therapists. If your PT provides descriptions of exercises, I believe there may be enough equipment onboard.