According to Asian boxing. He received a subdural hematoma from his last fight against Saludar and this will force him to retire. Great shame, Yamanaka definitely had some talent within him, but health is a bigger priority. Happy retirement champ!
Slow poke https://www.boxingforum24.com/threads/yamanaka-to-retire-due-to-subdural-hematoma.613429/ Massive shame given his youth and talent