Reus picked up the injury in February, and missed the rest of the season as Dortmund finished second in the Bundesliga.
However, having returned to light training towards the end of the 2019-20 campaign, there were hopes he would be back in time for preseason training, but as the club begins that Thursday, Reus is set for an extended spell on the sidelines.
“Borussia Dortmund will still have to be without Marco Reus,” the club said in a statement. “The captain has problems with tendon inflammation at the same injured groin muscle. He will be out for an indefinite period.
“Reus has worked intensively in recent months and has taken steps forward. But he still has problems with some movements.”
Dortmund sporting director Michael Zorc added: “Unfortunately the tendon is still inflamed. He cannot train with the team yet. It is difficult to give a time frame. This is the case with tendon injuries in the adductor region.”
Mats Hummels is also out injured for a short spell having twisted his ankle while training on holiday.