Hideo Kojima has revealed that Metal Gear Solid 5: Ground Zeroes will be a separate game to The Phantom Pain and act as prologue and tutorial. To go with the announcement are a bunch of new Ground Zeroes screens.
Speaking in a press release Hideo Kojima said, “The advanced capabilities of the FOX Engine have allowed me to tell the new story in a new way. There will be a significant difference in what Metal Gear Solid 5 The Phantom Pain brings to the series, so we want to ease players into the new open world environment and its potential. As such, Metal Gear Solid 5: Ground Zeroes has been designed to introduce key elements, allowing them to fully benefit from all that the new game will offer.”
You can read the full Metal Gear Solid 5 Ground Zeroes press release & get more info here.