Diaz collapsed after the 10th round of a nationally televised USBA welterweight championship fight against Delvin Rodriguez in 2008.
He was carried from the ring on a stretcher in front of his hometown fans and underwent emergency brain surgery after lapsing into a coma.
Diaz regained consciousness after two months and was released from hospital after seven months but failed to make a full recovery.
The San Antonio boxer, who had fought 31 times, had been living in a nursing home and could not walk on his own.
The Bexar County Medical Examiner???s Office confirmed Diaz died due to complications related to the brain injury suffered against Rodgriguez.