User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-6342763-20131016153208/@comment-11051098-20131016164644

Super side swiper is correct. In chapter 532, it was stated that Isshin's Bankai puts a strain on his body, so when he's injured, he cannot use it.


 * Quote from Aizen: "His Bankai is quite a burden on his body. He won't be able to use it with that wound."
 * Quote from Isshin: "This wound is deep.. I can't fix my reiatsu... At this rate, I won't be able to use my Bankai..."