First of all, it depends on your state. In Georgia, you can terminate him for applying for workers comp. Yes, strange but true. But almost everywhere else, you can terminate an employee who is on workers comp leave if they will not be able to return to work in a reasonably foreseeable length of time.
Most companies terminate an employee who has been out over a year.
Once they return to work, you must treat them as if they had never left. So if they are not performing, you can coach, counsel, discipline and then terminate. If their lack of performance is related to their injury, then you must accommodate under the Americans with Disabilities Act. As you can see, this is a complex question. Please see a local attorney before you terminate this person.