The 30-year-old sustained the injury in Wednesday's UEFA Champions League victory over CSKA Moscow, leaving the action after just 28 minutes.
Tests revealed tissue damage to his right thigh and the versatile defender will now be absent for Saturday's Bundesliga match against Eintracht Braunschweiger and the DFB-Pokal fixture versus Augsburg four days later.
Lahm will undergo further tests and Bayern will hope he can make a swift recovery, with several first-team players already sidelined through injury.
The German and European champions are without Bastian Schweinsteiger, Franck Ribery, Claudio Pizarro, Xherdan Shaqiri and Holger Badstuber.
Despite that lengthy list of absentees, Pep Guardiola's side are unbeaten since their Super Cup defeat to Borussia Dortmund in July.
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