Howard, who sat out the Magic's 109-81 loss to the Atlanta Hawks on Friday night, officially has been ruled out of Orlando's game in Cleveland on Sunday.
Including Sunday, Howard will have sat out six of Orlando's past eight games, a new personal high for missed games in a season (he missed four games last season).
Magic spokesman Joel Glass said Howard was examined by Dr. Robert Watkins after Howard flew to the West Coast earlier Friday for a second opinion for what previously had been called back spasms since he injured his back at the end of March.
There is no timetable for his return and it will be based on how he responds to treatment.
With the playoffs fast approaching, the reigning three-time NBA Defensive Player of the Year's absence leaves the Magic all the more concerned.
"Are you kidding me?" the Magic's Glen Davis responded, according to the Orlando Sentinel. "It's going to be tough without him next couple weeks. A herniated disk? That's crazy. Dwight isn't playing, everybody has to step up. Everybody."
The Magic (34-25) are 1-4 without Howard this season. Before this season, he had missed only two games in his career because of injury; the rest were either because of illness or suspension.
Magic coach Stan Van Gundy says Howard was in pain after the team's practice Thursday and the coach wasn't optimistic he'll be ready to play against Philadelphia on Monday either.
The Magic are in sixth place in the Eastern Conference playoff race.
Information from The Associated Press was used in this report.