Injuring the ACL in very common in athletes, especially female athletes. I've been working with female athletes over the past decade to prevent this type of injury.
What about when an ACL injury DOES occur? Rehab post-op is imperative! Just think, the clearance to RUN doesn't happen (or shouldn't happened) until at least 4 months post-op. Why? The body is still accepting this new ligament into the system and at 3 months out, you are most likely to re-tear it again.
What about a YEAR out? Although you're pretty much "back to normal" at this point, considering appropriate rehab, there are still findings of abnormality when compared to the leg that wasn’t operated on. Oh great, big deal? What does that mean?
If there are deficits from one side to the other (weakness, muscles not firing appropriately). Even a year out you may beed a tune up with a PT!
Why? As shown in THIS study... deficits were STILL found when walking up and downhill a year after ACL surgery.
This equals more wear and tear on that joint, leading to a higher arthritic joint, which then leads to other complications down the road (*cough cough* an early total knee replacement).
So, at the end of the day: an ACL injury will require you to see a professional who can help you not only in the short term, but in the LONG-term as well.