What makes you think that it's an ACL tear?
I have two 75 lb hunting dogs that are balls of energy and bull through brush and bramble like mike singletary filling the hole. My older dog, during the middle of hunting season, jumped down from the couch and landed wrong. He immediately started limping, wouldn't put much weight on it, and was licking it. My wife and I were both concerned, but it was Saturday afternoon and out Vet was closed. We decided to take our dog to the emergency vet to get checked out. They took x-rays, but they proved to be inconclusive relative to soft tissue injury, but appeared to rule out a break. They said that they could do an MRI, but wouldn't really recommend either way. We ended up deciding to keep our dog inside most of the time and guard against aggravating it, and schedule an appointment with our normal vet first thing in the AM. By the time we got to the vet for our appointment on Monday afternoon, the dog was walking around normal and with no evidence of pain. The vet didn't recommend any MRI or other treatment, but supported my wife's suggestion for taking a week off from hunting. In reality, the dog wasn't suffering from a serious injury, merely tweaked his ankle.
The moral of my story is that dogs, like humans, can get banged up a little here and there and its hard to tell whether its serious or just a "walk it off" type situation. I would recommend taking normal precautions, schedule an appointment with your vet, but don't panic too quickly thinking your dog tore an ACL or has another injury. That being said, if your dog has severe pain, or is in a situation where quick attention may be important, then don't hesitate to seek help immediately.
I hope that your dog is OK...