Actually it tells us your doctor knows a lot about what happens next. In the months after a hip replacement you'll be busy with healing, rehab, and recovery. That's not the best time to do something about a possible leg length difference.
About three to six months after surgery you should start to see a relaxation of the hip and leg muscles. Some doctors won't even allow the patient to use a shoe insert or shoe lift until a full six months has passed.
Make sure your physical therapist knows this is a problem. The PT can often help bring about a change through stretching and/or strengthening exercises. Ask your doctor again what can be done if there's still a problem at your six-month check-up.