First you need to read your post. Does it indicate in anyway that you had seen a vet? Does it indicate in anyway that you need a second opinion?
Lastly, this service is to provide help, it does not replace the need for a veterinarian...I can't see your dog, see the limp, see an xray...I can only go by the information that you provide. Any lack of information is on your part.
Now, if your dog has ruptured a cruciate ligament...and that has been diagnosed by your veterinarian than surgery is the recommendation. Do you have to do surgery...no. Eventually the knee will develop enough scarring that it will stabilize...but the dog will always have a limp.
If this was not diagnosed, you are correct in that it needs to be...if the dog is young, which I don't know because you didn't tell me, then there are things that could be contributing to the limp as well. These dogs are prone to OCD lesions where the bone is soft in the joint.
In any case...if there is more information you can provide to assist me in helping you, please do so. If not and you just still need a diagnosis/second opinion...then maybe you need to find another vet in your area to see your dog.