It's unclear what the exact diagnosis is, but Antetokounmpo said tests conducted by Bucks personnel in China confirmed he wouldn't be able to play.
The Greek Federation was not happy about the decision, calling it an "Organized and well-planned project by the Bucks," per Sportando.
The Greek Freak has yet to miss more than five games in a season since he debuted in 2013.
Last year, he averaged career highs of 22.9 points, 8.8 rebounds, 5.4 assists, 1.9 blocks and 1.6 steals en route to Second Team All-NBA and Most Improved Player honors.
Antetokounmpo will now shift his full attention to working his way back to full strength in time for the start of the 2017-18 season.
Milwaukee will open the fresh slate Oct. 18 at TD Garden against the Boston Celtics.