If your dog has a partial cruciate ligament tear, your dog can naturally heal in 6-12 months as scar tissue will form. During this period, a knee brace is recommend to firstly, provide extra support and promote healing, secondly, to reduce the risk of further injury e.g. turning quickly.
If the knee injury is a full tear the only way to fix a full tear is through surgery. If surgery is not an option, perhaps your dog is too old, a knee brace will be a lifetime commitment for the dog, meaning the remainder of the dog’s life.
An MRI scan will show if it's a partial or full tear, as well as looking during surgery.