Your doctor may be waiting for the inflammation to calm down. When a ligament is torn or partially torn, the body responds immediately to help the healing process. Sometimes this results in a lot of swelling in the area, with a loss of knee motion.
Your doctor may have you start a physical therapy program before surgery. Physical therapy is used to help reduce pain and swelling and improve knee motion and strength. Once the initial healing reaction has calmed down and you have your motion back, surgery can be scheduled. Research shows improved results when ACL surgery is postponed until these goals are achieved.