The legendary goalkeeper commended BVB for their performance in Europe's elite competition, but believes the lack of strong bench has harmed their league title defence
BVB topped Group D of the Champions League ahead of the likes of Real Madrid, Manchester City and Ajax, but find themselves just third in the league standings, 12 points off leaders Bayern Munich.
Kahn complimented Jurgen Klopp's men for their impressive campaign in Europe's elite club competition, but stated that the rigours of playing on three fronts has exposed Dortmund's lack of depth and has resulted in the club's recent struggles in the Bundesliga.
"BVB played out an impressive Champions League group stage, which has to do with the fact that the team focused heavily on the competition," Kahn wrote in his regular column for Bild.
"In the Bundesliga, however, Borussia were missing the necessary concentration. The gap between their performances in the Bundesliga and the Champions League shows that quality in depth is needed in order to play at a top level in both competitions."
Kahn also took time to comment on the dismissal of Schalke coach Huub Stevens, and backed Mainz's Thomas Tuchel as the perfect replacement for the Dutch trainer.
"Many fans feel the firing of the coach was an error, but I disagree as much as I value Stevens," he said. "Now that the club and the coach have parted ways, there is a chance to set the side on a new course.
"Stevens' successor should have the right to choose and implement a club philosophy. Mainz, for example, have a very good coach, who is allowed continuity and systematic work, and he has made his mark.
"I imagine that a coach like [Thomas] Tuchel could do very well at Schalke."