Yeah, if your arm is okay with forehands, you can do some basic forehand strokes. If doing backhands hurts or really doesn't work, don't do them. Only do them if they feel good.
But table tennis is not physical therapy. Physical therapy is work, movements, exercises, strength training, stretching, and electric stimulation, done in a controlled environment to target the kind of rehabilitation the area needs. For instance, if you had a knee cap that was not tracking right because the outer quad was to strong and tight (common) and the inner quad was to weak and flexible so the patella was tracking laterally, you would stretch the outer quad, strengthen the inner quad and do a bunch of other movements that help the patella track properly.
Whereas, the strokes in table tennis are continually adjusting to different placements of the ball and the movement is not controlled because you are adjusting to a moving ball. Sometimes you are hitting the ball when it is closer to you. Sometimes the ball is a bit farther from you because you did not move your feet to the exact spot. So, while you are using your arm and moving it, this kind of action where you are adjusting to a variable like the trajectory of the ball, would not be conducive to therapeutic exercise which is designed to make a body part that is not functioning properly, function well enough to do a more variable kind of action like playing table tennis.
Say you had a baseball pitcher who injured his elbow. He would not pitch to rehab his elbow. But he would probably do a bunch of therapeutic exercises to get his elbow and the surrounding muscles in shape so that he could eventually go back to pitching.
For now, if you hit a table tennis ball, only actions that do not hurt the arm. First rule: CAUSE NO HARM. As your arm gets better you will be able to do more. But table tennis is not what will get your arm better so that you can play table tennis. The correct therapeutic exercises for your injury is what you would want.
