The 24-year-old has now won three titles in a row, following two Bundesliga crowns with Borussia Dortmund, but says he was not as effective this season due to injuries
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."