Correct. As I stated in my post, they can REQUEST to get a second opinion, but can't just go out on their OWN. They have to consult with the Team Doctor first.
Kyle said the same thing. You can go to an outside doctor but it is very much discouraged. Or at least it was with him - when he was on the Rams. He was told that he had the best doctors, who were specially trained to help NFL athletes, that could do a much more thorough job than these "outside" doctors.