Sahin left Dortmund's training camp in Switzerland on Thursday to have his knee checked in Munich and the Bundesliga club later confirmed on their website that the Turkey international will miss three weeks of training to recover.
The 25-year-old midfielder played 45 minutes in Dortmund's friendly versus Rot-Weiss Essen on July 27 - wearing the captain's armband - but has apparently been struggling with pain in his knee for some time.
The knee problem will see Sahin miss the DFL-Supercup against Bayern Munich on August 13, as well as Dortmund's DFB-Pokal tie against Stuttgart Kickers two days later and his team's first Bundesliga fixture of the season against Bayer Leverkusen on August 23.
Sahin played 34 Bundesliga matches for Dortmund last term, starting 31 and scoring two goals.