Kagawa burst onto the scene at the start of last season after joining Dortmund as a relative unknown from Cerezo Osaka, scoring twelve goals in 27 starts in the first half of the campaign.
However, the 22-year-old suffered a serious injury while on international duty with Japan at the 2011 Asian Cup, ruling him out for the majority of last season.
And while he has since returned, Kagawa has looked a shadow of his former self as he bids to regain full match fitness and sharpness.
Following another underwhelming performance in Japan's 1-0 friendly win over Vietnam on Friday, Kagawa admits he is currently undergoing a difficult time in his career, although he is hoping to learn from it.
"I think this is the most difficult point in my football career but I will definitely get through it," Kagawa told Kyodo News.
"This is what I need for me to grow as a player and at this point just scoring one goal won't change anything.
"I know that not everything is easy."
Kagawa also reveals his belief that his current problems are of a mental nature, suggesting that while he is back to full fitness, he needs to rediscover his confidence.
"I'm suffering because I'm not delivering results. It's easy to play with joy when everything is going well.
"It's a mental problem that I have at this moment. In order to become a better player, this is what I have to go through.
"Last season I was able to find confidence and this confidence I have will not go away that easily.
"I am suffering now but I know that is going to change."
Kagawa will be hoping to make more of an impact in Japan's next match, a 2014 World Cup qualifier against Tajikistan scheduled for October 11.