Gundogan has not featured for Jurgen Klopp's side since the opening day of the season, having suffered the problem while on international duty in August.
Despite reported interest from Manchester United and Real Madrid, Gundogan remains focused on his recovery, though he conceded that a decision would soon be made on his future, with his contract due to expire in 2015.
"I will soon make a decision that will be carefully considered," he told Sport Bild. "As I said, it's still really open.
"My priority is my quick recovery. Nothing is decided yet. Borussia is my first point of contact.
"I'm trying to be healthy and bring my performance on the pitch. This is important and a top priority for Borussia and for me."
Dortmund slipped to fourth before the winter break, having won just once in their last six Bundesliga matches.
Gundogan's return would represent a significant boost for them, and the former Nuremberg man, who is still confident of featuring for Joachim Low's side at the FIFA World Cup, feels his spell on the sidelines could prove beneficial.
"I believe that this phase has made me better," he continued.
"I'll come back stronger. It is still difficult to make a prediction about when I make a league game again but it can happen quickly.
"I am confident enough to say that I will fight for a World Cup starting place."
