Chapman is undergoing treatment with antibiotics for a knee infection, Susan Slusser of the San Francisco Chronicle reports Monday.
Chapman is still under observation by team doctors. The third baseman slid into the wall making a catch while on a rehab assignment at Triple-A Sacramento, and has also made some diving plays in Oakland, but the club and player are both unsure when or where the infection started. A timeframe for Chapman's return won't be available for at least 24 hours, so it seems reasonable to expect he will miss Tuesday's game as well.