User blog comment:Godisme/Spotlight Brocken/@comment-2089817-20110317130324

Actually, there is the chance that the Substitute was actually Isshin, and that when Tsukishime "killed" him, that's when Isshin lost all of his powers, and made it seem like he died. Of course, its been established that Isshin is pure blooded Shinigami, and from the look of the haori over his shoulder, that he was a Captain. The chances of him being a substitute are highly unlikely.