Dr. Chang was the physician of Milton Winters.
For many years Dr. Chang encouraged Winters, who at the time was overweight, to lose weight and exercise. Once Winters was incarcerated in Greenhaven Correctional Facility, the prison diet and time to exercise finally worked to improve Winters' health.
It was initially suspected that Winters might have wanted to have Chang killed upon discovering a secret hit list that he created. However, Detective Robert Goren realized that the suspected hit man was deathly afraid of traveling above or on the water, and would have been unable to travel to Staten Island to carry out any contract against Chang. (CI: Cuba Libre)