Gundogan has missed Dortmund's last two games after suffering the back injury on international duty, with the problem now likely to keep him sidelined until October.
The 22-year-old will now definitely miss Germany's World Cup qualifiers against Austria and the Faroe Islands next month, as well as Dortmund's opening game in this year's Champions League.
Dortmund confirmed in a short statement on their official website this afternoon that team doctor Markus Braun has ruled him out for another month.
The brief message read: "Due to a stress reaction in the lower spine the Germany international will need an additional rehabilitation time of approximately one month, according to BVB team doctor Markus Braun.
"The extent of the injury was discovered after an examination at the Knappschaft Hospital on Monday following the midfielder´s return to Dortmund."
Jurgen Klopp's side have made a fine start to the season despite Gundogan's absence, taking maximum points from their first three Bundesliga matches.