Manchester United star Shinji Kagawa has vowed to take better care of himself after being sidelined by a back injury that he said may have been caused by "travel and fatigue".

Kagawa, forced out of Japan's 1-0 win against Iraq on Tuesday by a minor back strain, admits he needs to strengthen his body to prevent further problems.

It is unknown when he will return for his new club.

"It may be due to travel and fatigue," Kagawa told reporters on Wednesday, before boarding the 12-hour return flight to England.

"I can get by if I take good care of my body," he added.

"I've learned a good lesson from this."

The attacking midfielder felt a "twinge" in his back during training and despite treatment, was eventually withdrawn from Japan's team just before kick-off in Saitama.

Kagawa, 23, has not been known to suffer any back problems since starting his professional career in 2006, with J-League side Cerezo Osaka.

But he said joining Manchester United in a high-profile summer transfer from German champions Borussia Dortmund had shown him he needed to improve his physique.

"After moving to the Premier League, I've realised that I need to limber up and strengthen my body trunk," said Kagawa, who is 172cm and 63kg.

Kagawa gave fresh reassurances the back problem was "not serious" and said he was taking medication.

"I think I can come back (and play) soon. I have to consult my medical staff and do my best until the next match," he said.

Manchester United boss Alex Ferguson may be unlikely to risk his new signing against Wigan Athletic on Saturday.

The Red Devils also play Turkey's Galatasaray in the European Champions League next Wednesday.

"Tough games are going to start and I will have to fight for a regular spot. I want to go out in perfect condition," Kagawa said.