After enjoying an impressive start to his United career, triggering 'Kagawa-mania' across Japan, the forward failed to mask his disappointment after Thursday's match.
"Of course I wanted to play more minutes," the 23-year-old told reporters after Japan struggled in their final warm-up for a home World Cup qualifier against Iraq on Tuesday.
"I always want to score but the coach makes the decisions. I'll have to look at my performance today," added Kagawa, who sat at the end of the bench in the second half, barely talking.
"My body felt a little heavier than I expected, probably from the flight."
Kagawa has been a instant hit at Old Trafford, earning praise from United manager sir Alex Ferguson, who paid 17 million euros for the former Borussia Dortmund playmaker in June.
His transfer to one of the world's top clubs elevated him to celebrity status, leading to him being mobbed by fans at Sapporo airport before a friendly last month.
However, Kagawa was ineffective on Thursday, striker Mike Havenaar sparing Japan's blushes with a 69th-minute winner to give the Asian champions an unconvincing win.
Midfielder Keisuke Honda had blazed over with barely a minute on the clock following a clever backheel from Kagawa but that early promise was the United signing's best moment in a tired-looking display.
Kagawa often found himself taking the ball into blind alleys and was hauled off after frustrating coach Alberto Zaccheroni by attempting a few too many stepovers which led to no end product.
"Fatigue is not an excuse," Kagawa said. "I wasn't able to run as much as I thought I would. I gradually want to focus my sights on the Iraq game. The next game will be a completely different atmosphere."
Captain Makoto Hasegawa and Honda were also substituted early on Thursday ahead of the clash with Iraq, currently under the stewardship of former Japan coach Zico.
Japan overpowered Oman 3-0 and Jordan 6-0 at home before being held 1-1 in Australia in June in their first three games in the final round of Asian 2014 World Cup qualifiers.
"The next game is the most important. It will be a lot tougher - it's why I'm here," Kagawa added. "Hopefully I can make a real difference and we can take our chances."
The Blue Samurai lead World Cup qualifying Group B by five points from Iraq, Australia and Jordan.
