User blog comment:Xilinoc/Ch. 571 (Yachiru's gonna wreck it)/@comment-5391752-20140306025638/@comment-24282510-20140306030824

No remember both Sajin Komamura and Retsu Unohana (well her more so then Sajin) shikai seems to materialize another body. While both Sajin and Retsu did it after they had already became captains to us they still had to gain shikai to get bankai. We also don't know if those are what their spirits look. Plus just materializing is not enough to gain bankai they got to be beat into submitting.

That being said its a zanpukto technique not spirit materialization.