Orochimaru's mystery description says "Kill the ninja with the lowest life from your own team or adopt a defeated ninja and summon Hokage 1 then 2 with reanimation jutsu"
So the ninja Orochimaru chooses to use to resurrect doesn't have to be dead yet. If the lightning main is the only remaining alive ninja (I assume besides orochimaru), even if he's at mostly full health he can be replaced with a kage.