Poland international Lewandowski, 28, signed a five-year deal when he joined Bayern on a free transfer from Borussia Dortmund in 2014.
Leverkusen crashed out of the DFB Pokal on Tuesday after losing in a penalty shootout to third-tier club Sportfreunde Lotte.
Schalke coach Markus Weinzierl has praised Tottenham loanee Nabil Bentaleb, but did not confirm they have already triggered an option to buy.
Badstuber received a rapturous reception when he came on to replace fellow Germany international Mats Hummels.
Borussia Dortmund midfielder Mario Gotze told reporters that his side's narrow defeat of Union Berlin did not go according to script on Wednesday.
Carlo Ancelotti praised predecessor Pep Guardiola for the Bayern Munich side he inherited as the German Cup holders moved into the DFB Pokal last 16.