User blog comment:Xilinoc/Ch. 573 (Let Unohana return plz)/@comment-24153155-20140320091659/@comment-5398287-20140320210402

If you're referring to her Bankai disappearing, that actually happens when a Shinigami is near death, as noted by Byakuya when Renji's Bankai vanished after Byakuya nearly tore him to shreds with Senbonzakura Kageyoshi. However, Renji managed to survive that, so Unohana MAY have. Still, her death seems pretty certain at this point, unless Kubo is going for the "Kenpachi is lying/was fooled by something/didn't properly sense her power" route or something.