Thats a well known problem my friend.
First of all, for your own good, you should listen to your doctor and let it heal. But yet, you can do some exercises that wont hurt you and still be able to help a bit.
1. Cycling. For your physical fitness. There is no pressure on the knee at this way so it is a good replacement for whatever physical fitness you used to do.
2. Some basic exercises of hitting the ball with your bat, and those kind of exercise. It might not help you improve, but its defenetly helps you stay in shape a bit.
i use those exercises too since i do not often see a tt table nowadays. Always good to keep a bat and a ball nearby
here is a video with the exercises i was talking about (and if you be creative, there is much more you can come up with to help you train):
http://m.youtube.com/watch?v=i8Pv8vWdyL8