hmmm...
How experienced is your Vet? From you first post, I am not sure the Vet is experienced enough...
Yorkies are prone to Luxating Patella (loose knee caps) and/or Legg-Perthes disease (hip). Most Vets can determine if one of these are present.
If it is an ACL issue, I think you need to contain the dog to rest and hopefully it can heal on it's own. Otherwise surgery is needed.
I do not know too much about Rimadyl, but I have a weird feeling about it. Do a search here on YT for it ans see if others have had issues with this drug
