It is common to have a stiff knee in the morning after surgery due to the natural inflammatory process and swelling. The best way to reduce stiffness is to walk around for a few minutes or ride a bike for a few minutes first thing in the morning. You also might try to do your home exercise program given to you by your physical therapist to help reduce soreness as quickly as possible. The soreness and stiffness will gradually improve each day and should be gone within 3-4 weeks.