The Japan international has made 24 appearances in all competitions this campaign, helping United to win back the Premier League title in the process.
But the 24-year-old, who joined from Borussia Dortmund in June 2012 before being beset with injury problems, is dissatisfied with the role he has played.
"I was looking to produce even more results," said Kagawa, who did manage to score a hat-trick against Norwich City in March.
"I was injured and I felt things didn't go well easily. I had a lot of conflict, frustration and worries.
"In that sense it has been a profound year.
"Honestly I haven't contributed as much as I did at Dortmund. I still don't quite have that feeling of accomplishment.
"But I've managed to win three [titles] in a row. I think my career is bound to get better and better."