Japan have been dealt a huge blow in the lead-up to Saturday's Asian Cup final against Australia.

The Blue Samurai have lost key midfielder Shinji Kagawa who broke a metatarsal bone in his foot during the semi-final triumph over South Korea on Tuesday.

Initially the Japanese officials were hopeful the Borussia Dortmund attacking midfielder would be right to play in the decider but scans have revealed the extent of the damage and he may even miss the remainder of the season with his German club according to his agent.

"Shinji has sustained a metatarsal fracture and is already on his way back to Germany to undergo further treatment. He could be sidelined for the remainder of the season," agent Bernhard Schmittenbecher told Bild.