Just saw your post and thought I'd chime in. My Bullmastiff Porter is in a similar boat. I had to go on a business trip back in early November, and the second day I was gone the place I board him called and asked if I'd noticed him limping. I hadn't of course, I'm guessing he hurt it playing with the other dogs. I kept him restricted in activity for about 3 weeks myself, and as it wasn't getting better I took him into my vet. He examined him and said he suspected it was a cruciate tear, we put him on anti-inflamatory meds and restricted activity again. 2 weeks later and he hadn't gotten better. I did some research myself, though oddly never thought to check here. I found some of the same sites the others had mentioned. In the end I decided to go with the surgery. Porter is scheduled for the end of next week, so I'll be sure to report back in after a while as his recovery progresses. Good luck with your pooch, he's a good lookin guy.