The 24-year-old Japan international said: "It was a tough time for me, with a lot going on in my head.
"I am not happy with my performance for the team at all. I didn't score that much and I also had my (knee) injury.
"Despite that, it was great we won the league, but I just don't feel that I achieved everything that I wanted.
"For me personally, it wasn't a good season."