Despite picking up a league winners' medal as United secured their 20th title the Japan international was not satisfied from a personal point of view.
"It was a tough time for me with a lot going on in my head," said Kagawa of his first season after his move from Borussia Dortmund.
"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, I just don't feel that I achieved everything that I wanted.
"For me personally, it wasn't a good season."