The defensive leader's continued absence represents a major blow for Dortmund, who have also had to contend with club captain Emre Can remaining in the treatment room throughout the summer.
Commending the dedication of both senior figures during their recovery periods, Dortmund sporting director Ole Book explained: "I see both of them fully motivated every day. During their rehab, they both contribute enormously as leaders - to the team, to the togetherness, to the group. But how long it will take exactly, nobody can say - and certainly not me."