He had vertebrae scraping against each other, and it was only continuing to get worse. His career was basically over, and his one shot was major surgery that actually had a pretty high risk-factor (as in, if something went wrong he could have ended up with partial paralysis).
It seems to have gone well, so now he's just got a long road of physio ahead of him. If you're a fan, cross your fingers for him.
Edited to Add: No one's ever confirmed this, but I think his back got screwed up when Rich Franklin folded him in half with that right cross.
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