Well, surely the medical officer must have the right to make an examination of all officers if he or she doubts the fitness for duty. Making McCoy say that he hadn't had a chance to examine Decker, and therefore preventing McCoy from relieving Decker of command, is a catch-22, isn't it? He can't relieve Decker of command without an examination, but he can't examine him without relieving of him command. On the face of it, it makes McCoy powerless.
There must be a regulation, even if TOS, that would allow an officer to be relieved due to medical unfitness without necessarily examining him or her first.