In all NA team sports, a sick/injured player can be substituted for, and he can re-enter the game if he recovers, except for baseball, which doesn't allow substitution period. In soccer, he can't be substituted for, but he can try to recover on the sideline while his team plays a man short.
BTW, an injured baseball player can receive any kind of medical treatment while his team is not on the field!
In golf, a player can get any kind of treatment from the tour physio, as long as he doesn't hold up play. If tennis implements no MTO, it'd the only sport who doesn't provide an opportunity for a sick/injured player to recover! That would make no sense! There will be players who try to play through crippling injuries, because they're unsure about the severity. At least now, they can receive professional medical from qualified doctors/physios.
Tennis should implement this rule: if a player gets to see a physio, his/her opponent gets to see the coach. As of now, the opponent gets to suck on his thumb, and gets all cold and nervous when play resumes!