As long as there is no evidence of articular
cartilage damage on MRI (or at your arthroscopy), you may not be at high risk for degenerative osteoarthritis.
Weight loss is critical if you expect to continue playing tennis. Degenerative osteoarthritis is not usually caused by a single injury. It usually is the result of small micro traumas over time.
Lose weight and there is less force on your articular cartilage every time you step or jump.
Jumping around on the court with small spit steps may actually be good for your your knees!
When you go down into a split step, you are using your leg muscles to absorb the shock.
Something I've noticed: those who are overweight don't split step and hop around the court, absorbing the force with their legs. They land with one thud on their knees, and don't let their body weight fall over a distance.
Tommy Haas practicing at 2009 LA tennis http://www.youtube.com/watch?v=u6avhDgY1l8
The squat is actually a great exercise to protect your knee. It is a closed chain exercise, so there is no "lever action" eccentrically pulling on your knee like a leg extension.
Squats strengthen every leg muscle, plus your core muscles, plus the muscles that connect the legs to the core.
Walking lunges are a great way to take the strength gained from lunges and make it into usable strength for better movement and to better absorb shock at the knee.