Would this really be an upset? The way I see it is, whichever one lands cleanly first wins. It's more like 50/50.
I think in terms of MMA it would be quite an upset, because Dos Santos is, and I think rightfully so, considered the much better MMAist, as, even though he wins basically all of his fights with his boxing, he knows much better how to avoid the ground and thereby is better equipped against wrestlers and grapplers. But considering that both fighters have their offensive strengths mainly in the striking department and probably will have a striking contest, it wouldn't be that much of an upset if Hunt comes out on top with landing cleanly first as you say.
So on paper it would be an upset, but as Dos Santos has never really wanted a fight not to be a pure striking/boxing match, a Hunt win is pretty thinkable.