The Bucks announced Thursday that Lopez had back surgery earlier that day in Los Angeles. Team officials didn't provide a timetable for his eventual return.
Lopez played 28 minutes and had eight points, five rebounds and three blocks in a season-opening victory over the Brooklyn Nets, but the 7-footer hasn't played since. The 33-year-old center is in his fourth season with the Bucks and 14th season overall.
He averaged 12.3 points, 5.0 rebounds and 1.5 blocks last season while helping the Bucks win the NBA title.
Bucks officials said Robert Watkins performed the surgery, with oversight from team doctor William Raasch.