I had rotator cuff surgery three weeks ago. I'm still in an immobilizer and with a special pillow under my arm. I'm not supposed to move my shoulder unless the physical therapist does it for me. I'm really worried about getting a frozen shoulder. What are the chances of this happening?

Your post-operative care is prescribed by your surgeon and depends on the kind of surgery you had. The type of injury, size of the tear, and method used to reattach the tendon
direct what positions and motions you can use. You don't want to do anything that will disrupt the repair, especially during the early weeks of tissue repair.

It's true the shoulder can scar down and get "stuck" or "frozen" if you don't move it. The abduction pillow you are using along with the physical therapist's range of motion
exercises should keep you from developing problems.

Many researchers advise following a program just as you've described. This will protect the repair from repeated loads it can't handle. With the help of your therapist, you'll be able to make up any lost motion quickly once you pass this early rehab phase.