Gundogan has featured just once for Dortmund this season after suffering a stress fracture in his lower back, a problem he aggravated further during the early weeks of the campaign.
The 22-year-old was initially expected to spend six weeks on the sidelines and return to first-team action in early October, but Zorc has admitted that the playmaker is still not back in full training.
He told Kicker: "It drags on. Ilkay is far from getting back to football-specific training."
Jurgen Klopp's side have coped well without the German international in the Bundesliga this season, topping the table alongside Bayern Munich with 19 points from a possible 21.