Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130622005445

"I understand why you no longer possess reiatsu - it's not that you *lost* it - you *discarded* it - I do not know what kind of method you used - but you converted all of your reiatsu into physical strength"

I went back into the chapters, and this is what Aizen said. I do however see that you seem to be right. It is a good point, stopping Aizen's blade bare-handed and destroying an entire mountain does show that he should have loads of reiatsu.

In a way not shown, it seems he gained so much reiatsu that even Aizen was unable to feel it.