In strains or minimal tears the ACL tear can cause temporary pain and lameness that can heal as long as additional injury does not occur. Rest with just enough medication to make the dog non-painful but not so much as to make him feel to good so that he uses the leg and potentially cause a more extensive tear. Some pain is your dog’s friend to remind him that he is still injured. If this pain is totally removed he will use the leg and unfortunately in many cases cause a more severe injury. After about one week of relative rest and leash walking to go out progression to leash walking for exercise for an additional one to two weeks would be reasonable. If however he begins or continues to toe walk or limp at any point than immediate additional treatment would be indicated.
If nothing is done at this point then you can expect at minimum progressive arthritic change. Further neglect may result in further tear of the ligament with more pain and disability and finally a second injury to the opposite leg because of the off-loading to the opposite leg. This occurs in 40 to 60% of dogs. Once a dog shifts its weight to the other three legs and rests the injured leg, it is not uncommon to observe your dog walking somewhat normally for short periods of time even up to a week and then relapsing to his original degree of lameness.