Schmelzer was withdrawn at half-time of Dortmund's 2-0 DFL-Supercup victory over arch-rivals Bayern Munich on Wednesday.
And tests have revealed that the 26-year-old sustained a torn abductor in his groin, meaning he will be sidelined for Dortmund's Bundesliga opener at home to Bayer Leverkusen on August 23.
A statement published on Dortmund's official website on Friday read: "Borussia Dortmund will be without Marcel Schmelzer in the coming weeks."
In more positive news for Dortmund boss Jurgen Klopp, both Oliver Kirch (foot) and Sokratis Papastathopoulos (knock) trained on Friday after picking up minor problems against Bayern.
However, it seems unlikely Klopp will risk the duo for his side's DFB-Pokal trip to third-tier Stuttgarter Kickers on Saturday.