User blog comment:Xilinoc/Ch. 616 - Mimihagi-sama/@comment-24531362-20150219192237/@comment-1604252-20150219220213

That's actually a good theory. But I have a feeling Ukitake has an actual Bankai. Chojiro was stated to have mastered Bankai before Ukitake and Shunsui, so I guess we can take that as semi confirmation.