Kagawa arrived from Borussia Dortmund in the summer, but found himself on the sidelines for two months of his debut year at Old Trafford due to a knee injury.
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"I was looking to produce even more results," he told reporters. "I was injured and I felt things didn't go well easily. I had a lot of conflict, frustration and worries. In that sense, I think it has been a profound year.
"Honestly, I haven't contributed as much as I did at Dortmund. I was even injured. I still don't quite have that feeling of accomplishment."
Kagawa showed flashes of brilliance in his debut season at Old Trafford, including a hat-trick against Norwich for three of his five goals this campaign.
And after sealing his third championship in a row, following two consecutve two Bundesliga titles, the Japan international believes he will only improve from here.
He added: "I've managed to win three in a row. I think my career is bound to get better and better."