The playmaker aggravated a niggling back injury on international duty during Germany's friendly with Paraguay last month, with an initial prognosis ruling him out for two weeks.
Gundogan has now conceded that he remains in the dark over when he can return to Jurgen Klopp's starting XI, but has vowed to work hard for a return to full fitness.
He told Bild: "I'm trying to get fit again as soon as possible. I don't not want to say any more."
The 22-year-old has become a firm favourite at Dortmund since signing from Nuremburg in 2011, making 84 appearances in all competitions.