Per Serena Winters of NBC Sports Philadelphia, Simmons will have surgery "in the coming days to remove a loose body in his (left knee)."
Per ESPN's Adrian Wojnarowski, a timetable for Simmons' return has yet to be determined, though he noted a postseason comeback is unlikely for the 24-year-old:
Adrian Wojnarowski @wojespn
While the Sixers haven't ruled Simmons out for the season, sources say it would take a deep run into the playoffs to keep the door open on the possibility of a return --- and that's still an iffy proposition. For now, the Sixers need to prepare for a postseason without Simmons.
Simmons has dealt with his share of injuries already this season.
When the NBA campaign resumed, Simmons did play in each of Philadelphia's first three games. He injured his knee during the third quarter of the Sixers' 107-98 win over the Washington Wizards on Aug. 5.
An MRI revealed Simmons suffered a subluxation of the left patella and would miss time while going over treatment options.
The two-time All-Star missed eight straight games from Feb. 24 through March 11 with a nerve impingement in his back.
Philadelphia has gotten accustomed to playing without Simmons this season, but its chances in the Eastern Conference playoffs look much better if he's in the starting lineup. The Australia native is averaging 16.4 points, 7.8 rebounds and a team-high 8.0 assists per game.
Until Simmons returns, head coach Brett Brown will likely turn to Al Horford as his primary power forward.