Yesterday Kubo stated on his blog that if he loses in his upcoming bout, on August 21st against Ryohei Takahashi (6-1-1), he would retire.
Aged just 26 Kubo has been involved in combat sports for years. It seems his long and hard career has taken it's toll on him and he now feels like his body may be done. It'd be a shame to see him walk away from the sport, though there is every chance that he needs a break and that a loss to Takahashi in August will not be the end of Kubo's career.
Notably a win for Kubo against Takahashi would give him a Japanese domestic ranking and possibly kick start his career which has faltered in recent years,