Very tough question.
First, x-rays usually are fairly conclusive for a tear because there is visible joint effusion (fluid in the joint) present. Second, if done correctly, you can feel cranial drawer. So, i would suggest having those x-rays reviewed by a radiologist if not already done!
Great that no limping, but NSAIDs can be very effective. Some dogs will not limp while on medications but as they where off, repeatedly worsen.
In fact, many times, people get confused with ACL tears because if they wait about 6-8 weeks the dog will stop limping. That is a direct result of scar tissue developing and giving a false impression that the joint is more stable. These pets will notoriously resume limping again a month or few months later.
But, on the brightside....if not limping while on NSAIDs, that means you can medical manage the condition at least for some time, and don't have to jump straight to surgery. In addition to the NSAID, I would ask your vet about also having at home, on an as needed basis, a regular pain mediationl like Tramadol or Gabapentin.
Hope that information helps