I'm pointing out the nature of the mission and how real-life medic specialties work. If you're not interested in having a discussion, just agree to disagree.
Since this would be the third (fourth?) time I've pointed out how easy it is to invent justifications and yet you persist in doing just that, I don't think its me who lacks interest.
So, again: my point here is how easy it can be to simply invent some justification. But this plot you've now created is jarring separate from everything we now. Its plainly implausible that MSF has a soldier better than Big Boss who has been unseen, unheard, and unspoken of until MGS V, and who's sole function appears to be as a minor field medic.
Even your supplied answers make little sense. If we suppose he's there in case the helicopter goes down, it now makes even more sense to have five or six guys in the chopper ready to go. Or to send two or three choppers. And on and on we can go.