The 22-year-old midfielder had been expected to finally return in early October but sporting director Michael Zorc warned he faces a longer absence.
"It drags on," Zorc confirmed to kicker.de.
"Ilkay is far from getting back to football-specific training."
Gundogan has not played since being substituted in Germany's 3-3 draw with Paraguay on August 14 when the injury flared up.
As well as Dortmund's forthcoming games, starting with Tuesday's Champions League clash against Marseille, he will miss Germany's World Cup qualifiers against the Republic of Ireland and Sweden later in October.
Although a key player for coach Jurgen Klopp, Dortmund have not missed him in an unbeaten start to the Bundesliga season which sees them top of the table on goal difference from rivals Bayern Munich.