You may not need surgery just yet. Studies show changes occur in the way the quadriceps (thigh) muscle works after ACL injury. Just as you report, the vastus lateralis (VL)--the part of the muscle along the outside of the thigh--is affected the most.
One group of researchers studying this problem tried using a knee brace to delay the VL response. It doesn't work for everyone, but it may be worth a try. In that study about 70 percent of the patients had a favorable response by using a functional knee brace.
Rehab and exercises to improve quadriceps control is very important. Biofeedback may be very useful in this program. Challenging your balance in all directions is also helpful. Ask your physical therapist for more help in these areas.