The Polish international, who last played in the Bundesliga in January before he was sidelined with cruciate knee ligament damage, is set to miss around four weeks of action.
The 28-year-old posted the following message on his Facebook page, which reads: "Unfortunately it's been confirmed: Kuba has suffered a muscle tear and will be out for around four weeks."
Kuba and Reus could both miss five Bundesliga games in that time, as well as two UEFA Champions League group games at home to Arsenal FC and away at RSC Anderlecht. Head coach Jürgen Klopp is already without Nuri Sahin (knee), Oliver Kirch (thigh) and Dong-Won Ji (hamstring), while Mats Hummels has not yet returned to training after several niggles ruled him out of Germany's 4-2 win over Scotland last Sunday.
There was more encouraging news for Italian striker Ciro Immobile, however, with Dortmund confirming that the hip injury he suffered while playing for Italy on Tuesday is not as serious as was first feared.