Hirales hit Leal, 26, with a clean right hand to the head in the sixth round, and again in the eighth. Leal was able to beat the referee's count but immediately collapsed, ending the fight.
Although he was immediately examined by medical personnel and transported to a nearby hospital, Leal slipped into a coma. He was transported from Cabo San Lucas to San Diego, Calif., where he then died.
"This hurts a lot," said Leal's promoter, Fernando Beltran of Zanfer Promotions. "We knew that Frankie Leal was a good kid, a great fighter, a warrior in and out of the ring."
"As a company we took responsibility and tried to save his life as much as humanly possible," Beltran said. "Now we are united in the grief felt by his family and his boxing family. We will try to help, whatever is needed."
But some have asked whether Leal should have been allowed to fight at all, as the medical community learns more about the devastating and permanent impact of brain injuries. He was knocked out in March 2012 by Russia's Evgeny Gradovich and hospitalized, and went on to fight in five more bouts.