Injured Gundogan in the dark over return date

The Germany international aggravated a back problem against Paraguay on August 14 and the source of the spinal pain has yet to be determined

Borussia Dortmund midfielder Ilkay Gundogan has refused to be drawn on when exactly he will be fit to return to action, amid mounting speculation that the club's medical team have been left baffled by the exact nature of his injury.

The Germany international aggravated a back problem during Germany's friendly with Paraguay on August 14 and was initially ruled out until the end of the month.

However, according to Bild, the exact cause of the spinal discomfort has not yet been determined, meaning Gundogan is not in position to set a date for his comeback.

"I'm trying to get fit again as soon as possible," the 22-year-old told the German paper. "I don't not want to say any more."

Gundogan, who joined Dortmund from Nurnberg in 2011, had begun the season in impressive form, scoring a goal in the DFL-Supercup win over Bayern Munich.