With a long injury list already, and the start of United's Champions League campaign to prepare for next week, Sir Alex Ferguson can do without any more problems.
Although Kagawa does not believe his back problem is serious, it was enough to keep him out of the 1-0 win in Saitama.
And he is not sure how long he is going to be sidelined for.
"I felt a twinge in the back during practice when I was on the ball. I had some treatment but didn't want to risk causing the team trouble by playing," Kagawa told the Kyodo news agency.
"Obviously it's a shame I wasn't able to play.
"It's not that serious and I don't think it will keep me out for long."
Source: PA
Source: PA