I also have a lurcher, who had the same symptoms your dog, it was a cruciate ligament injury, & same as yourself I was told he needed to loose weight. Yet NO walkies ! for a couple of months ! so then the vet did the procedure on his back leg. But I have been told No off lead running! So now since March, I do let him of for short spurts, however the Vet informed me that the larger breeds are prone to lameness. Please go easy on the walkies, I know it is extra worrying as he has just 3 legs, but I was told with rest especially on a young dog, the limping more often than not eases then stops. My fingers are crossed that the outlook is OK for him.