The Manchester United midfielder has hinted that his lack of starts at club level is affecting his performances after scoring for Japan in a 4-2 win over New Zealand
The 24-year-old has struggled to nail down a starting place at Old Trafford under David Moyes, making just 18 appearances for the Red Devils this season, starting 14 games.
Nevertheless, he featured from the beginning for Japan in their 4-2 friendly victory over New Zealand at Tokyo National Stadium on Wednesday before being substituted after 79 minutes.
And, despite contributing a goal from the penalty spot as well as an assist, the former Borussia Dortmund star said that he felt weaker in the second half.
"It really had been a while," Kagawa told reporters when asked about playing from the beginning. "But I still had to play well and earn a good result whether I started or came off the bench.
"I didn't feel as strong in the second half so that's something I have to consider."
Kagawa supplied the pass for Shinji Okazaki's opener early in the game before winning a penalty minutes later, which he duly dispatched, to help his team to victory.
The midfielder said that he was "happy" with his contribution but warned that Japan must improve if they are to make an impact at the World Cup in Brazil.
"We were able to get an early goal so that helped me get into the game. I'm happy I was able to earn the penalty and convert it," he said.
"We all had tight schedules before the match but we'd talked about getting into the game and building our momentum quickly. [New Zealand] weren't able to get into the match quickly so it's great that we were able to score so early.
"We have to consider that we still gave up goals, and the goals we scored were thanks to our opponent's mistakes, so it's no question that our games at the World Cup will be much closer.
"In that sense we have a lot to improve on."