I have had experience with this with 2 dogs...both Boxers. It is caused by high impact, quick starts, stops, bounces and turns etc. Your description of your Rotti does indeed sound like a prime candidate for it.
Both of our dogs had partial tears...not all the way through the tendon. We tried going the route of no surgery for the first guy. I do not recall how old he was, but not too old, as he died (of Cancer) before he turned 5 years of age. We tried several weeks of rest for him, as best as we could. It just meant for him, not letting him out with another dog at the same time...no running, jumping etc. We also had him on Rimadyl for the pain and inflammation. It would come and go with him, and when it came, it was very painful to him. The reason we decided against surgery for him, was because they were going to peel back the hide and take muscle from his hip/hind quarter to make the new piece. We felt that it would cause him so much pain, with no guarantee that it would even work, so we opted out.
Our other Boxer, a smaller female, also tore hers. With her, we chose to have the surgery done, because by that time, they were successfully using catgut to repair/create the torn part of the ligament. We had to keep her relatively quiet for 6 weeks. At the time, we had a Boston Terrier too, and she needed surgery for a luxating patella. We took advantage of the situation, having them both done at the same time, so they would both be in pain and would not want to play with each other, rather than having one healthy dog, bothering the surgical patient. We turned them both into patients. We also had it done in Winter (we live in a VERY cold area of Canada) so they did not spend much time outdoors, other than to potty. Hubby and I were both working at the time, being gone nearly 14 hours a day, so they were crated a LOT! Anyway, her surgery was successful, and she never had troubles with it again. She lived until she was 11. I think she was probably around 7 when she had the surgery done.