Simmons will not play in Monday's game against the Atlanta Hawks, sources said.
Simmons left the Sixers' 119-98 loss to the Milwaukee Bucks on Saturday after tweaking his back in the first quarter. Sources had described Simmons as emotional upon leaving the X-ray room at the Fiserv Forum late Saturday.
There is some level of concern surrounding the possible nature of the injury, league sources said.
Simmons left the game after playing less than five minutes, during which he tallied five points and two rebounds.
He missed the Sixers' victory over the Brooklyn Nets on Thursday with lower back soreness.
Simmons is averaging 16.7 points, 7.8 rebounds and 8.2 assists per game for the Sixers, who are currently the fifth seed in the Eastern Conference.