Erling Haaland may miss the rest of the season owing to a hip flexor injury he suffered recently.
The Norwegian is the Bundesliga's second-highest scorer this season, trailing only Bayern Munich's Robert Lewandowski, but he didn't feature in BVB's win against Arminia Bielefeld at the weekend.
Dortmund manager Marco Rose previously said: "Erling will be out, we'll be without him for a couple of weeks even. He has a hip flexor injury which simply doesn't make it possible for him to play. It's the same thigh, but another location."
However, according to Bild, Haaland has 'torn a muscle bundle in the hip flexor,' preventing him from participating in full training for at least six weeks.
Dortmund would be dealt a tremendous blow if they were to lose their talismanic attacker for an extended period of time, especially with no obvious replacement ready to step up.
Following Haaland's nine league goals, Raphael Guerreiro is the club's second-leading scorer with three goals, followed by Giovanni Reyna, Jude Bellingham, Julian Brandt, and Marco Reus, all of whom have two.
Dortmund has had a strong start to the Bundesliga season, and they are only a point behind champions Bayern in the rankings, but that gap could widen in Haaland's absence.
They have big games against RB Leipzig and Bayern Munich coming up before the end of the year, and they still need to find a way out of their Champions League group, despite winning two of their three games thus far.