I have a feeling that Phantom Pain and Ground Zeroes are both a part of the same game. Kojima said that Ground Zeroes is the prologue to MGS5, but maybe he meant that in the same sense that the Tanker is the prologue to the Plant, and Virtuous Mission is the prologue to Snake Eater. He also mentioned something about being captured in GZ means breaking out of prison, so maybe one of the large scale boss battles leads to Big Boss being maimed and having to find his way out of this hospital.

or maybe it's a simulation, who fucking knows.