The 30-year-old centre-back initially got injured in a calf-muscle problem during a pre-season friendly in August.
Though he recovered enough to return to some action, the fresh toe issue has proved harder to shake off, enough that reports claim the swelling became so bad he "couldn't even fit his foot into his shoe".
Sule's limited involvement this season has underlined how injuries continue to derail his time at the club.
With only a handful of appearances under his belt in 2025, the recurring setbacks have only increased speculation about his future at Dortmund, especially as his contract runs down.
Coach Niko Kovac has acknowledged that while Sule is making progress in rehabilitation, the defender still "has some catching up to do," leaving no guarantee he will return before the end of the year.