I could tell you that it is NOT required. I could tell you that I have been able to successfully rehabilitate ALL my post op CCL repair patients without it. But I feel the best response to this question comes from the dog owner. This is what one of my past clients had to say about the subject:

Max’s experience: Surgical and Non-Surgical Recovery from CCL tears.
 
My dog Max was a 78-pound black lab mix who we adopted as a 4-year old. He was mild-mannered, goofy and affectionate. Although he wasn’t an “Alpha” type, he enjoyed being outside and active—long daily walks and ample play time at the park. 
Max was 6 ½ years old when he tore his right CCL. I opted for a TPLO because I thought it provided his best chance to remain active. Besides surgery, he had a handful of traditional physical therapy sessions, though no underwater treadmill exercise. Max recovered nicely and regained his mobility after a 6-month recovery period, but if I had been more knowledgeable, I would have insisted on regular physical therapy. It is as critical to post-surgical recovery for dogs as for human patients.
At 10 ½-years old, Max tore his left CCL. With more experience this time around, I was less eager for him to undergo surgery. I decided on a trial basis of “conservative treatment,” a non-surgical approach that advised exercise restriction, anti-inflammatory medications, physical therapy, and weight loss. Following these guidelines, Max gradually regained enough function in his left rear leg to remain active for another 3 years. 
The conservative approach served him well, without the risk of invasive surgery. 
Since its guidelines are also prescribed for aging dogs with arthritis, I continued to follow them even after the left CCL healed. I feared that a sedentary life for Max, without active use of his legs and hips, would have been a form of imprisonment. He continued to have monthly physical therapy until the end of his life, which I believe was critical to his health, both physically and mentally, in his last years. 
 
Written by Susan I.