The defender failed a medical at Middlesbrough over the summer, but expects to be fully fit next month after a rib operation.
"I'm super. I'm in training and have targeted endurance and leg strength. I am very confident that I'll return in October to the squad. And then with great momentum, I have been fully in individual training since mid-August," the 27-year-old told Focus Online.
"I am still a player of Borussia Dortmund. My situation does not correspond to the ideal situation for me or for BVB. But I would not waste a second and give everything in order as quickly as possible to be in top form and back on the pitch.
"I have had conversations with coach Thomas Tuchel and manager Michael Zorc and it became clear that I and BVB have no future anymore."