Not necessarily. It's true that the longer a person waits, the greater the risk of the knee becoming unstable. Once the anterior cruciate ligament (ACL) is torn, there is greater strain on the other soft tissue structures in and around the knee.
Studies have shown that repair of ACL tears can be successful whether done right away or years later. If you've spent the last two years in rehab you may even have a better chance of good recovery. The exercises may have increased your strength. This can give you a "leg up" in recovery, so-to-speak.