I believe if you demote him during the same cycle as activating the now healthy guy there is no demotion penalty, but let someone else chime in on that.
Also, the advantage of placing anyone injured on the DL is it allows the Medical budget amount to factor into his recovery so he can regain some ratings, providing your budget is high enough. If you just 'rest' an injured guy, his time will eventually allow him to play again, but his recovery will not have this benefit.