GGG drained or not. Floyd will be joining Pacquiao on the injured shoulder list the first time he uses the 'shoulder roll' defense.
There are only two ways that it could play out. Stoppage for GGG or UD for Floyd. I think the obvious size and power advantage that GGG would have will be evened out by the fact that he will have to stop Floyd to win. My guess is that Golovkin will find it just as difficult to land clean head shots on Floyd as all the other fighters he has faced. The big question will be whether Golovkins power can hurt floyds body enough to force him into making a mistake and leaving an opening.