ESPN's Ramona Shelburne reported Friday the Sixers are waiting for the "inflammation around the nerve [in Simmons' lower back] to subside" before determining a firm timetable.
Philadelphia head coach Brett Brown said in late February the team wasn't sure how long Simmons would be sidelined.
"I don't know," Brown told reporters a day after Simmons suffered the injury Feb. 23 against the Milwaukee Bucks. "It really is kind of, like, how long is a piece of string? Who knows? Who knows? Whatever the time equals on days, games, period of time, then we can talk more honestly as this thing shakes out. But as it sits right now, that's how I see the world."
The 76ers are also dealing with the absence of center Joel Embiid because of a sprained shoulder. His return should come quicker, with Shams Charania of The Athletic reporting Wednesday's game against the Detroit Pistons is the target date.
Yet, injuries to Simmons and Embiid, paired with the team's up-and-down play all season, have significantly reduced expectations after the Sixers entered the campaign as championship contenders.
Philly sits sixth in the Eastern Conference with a 38-25 record, though there's still room for movement up the standings over the final month of the regular season. Finishing fifth or worse would likely mean needing to win four straight series without home-court advantage to win the title.
The Sixers' championship odds, which FiveThirtyEight currently peg at 11 percent, would be greatly reduced if Simmons isn't ready to play when the postseason gets underway.
Philadelphia finishes the regular season April 15 against the Charlotte Hornets, giving the point guard about five weeks of recovery time in order to at least see some minutes before the playoffs.