We were faced with that situation a few years ago. He's a big mongrel that pulled up lame chasing a rabbit. Vet said he needed ACL surgery. But he was about ten years old then. We gave him this product instead:

http://www.foodscienceofvermont.com/index.php?l=product_detail&p=300365180

It worked wonderfully. He's 14 now and still ticking. That stuff really works. My wife takes it too when she has joint pain.

Give it a try.