Last black lab rescue Eliza had a torn ACL when we got her. The vet said it wasn't a candidate for surgery, especially since it didn't seem to be debilitating.
Every now and then she'd get excited and try and romp around, but I could see that it would bother her and she'd slow down.
She was a senior, gray muzzle and face so it was OK that she wasn't a young pup.
Unfortuntely she died of pancreatitis a few months ago.