The Japanese international joined the Red Devils last summer from German outfit Borussia Dortmund in a deal thought to be worth around £17m.
However, a knee injury has limited the 24-year-old to just 18 appearances in the Premier League and despite the fact that he has found the net five times during those matches, Kagawa has insisted that he was keen to have made more of an impact.
"I was looking to produce even more results. I was injured and I felt things didn't go well easily," he said. "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 was part of the Dortmund squad that won consecutive Bundesliga titles prior to his departure.