The Bucks announced Thursday that Lopez had back surgery earlier that day in Los Angeles.
"He's basically been making very, very slow but little progress from the beginning,'' Bucks coach Mike Budenholzer said before Thursday's game at Toronto. "We hoped rest and rehab and things like that would solve it and it hasn't.''
Team officials didn't provide a timetable for his eventual return but said "Lopez will continue to be listed as out and updates on his rehabilitation progress will be provided as appropriate.''
"We're not going to put a timeline on it, but we're also not going to say he's not going to play this season,'' Budenholzer said. "We feel good about the surgery today, and he'll begin his rehab and do everything he can to put himself in a position to play for us.''
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.