The No. 1 overall pick is expected to miss about three months, a source told ESPN's Jeff Goodman, which would peg his return in December or possibly even January.
Sources told ESPN on Saturday that doctors involved believe Simmons suffered an acute injury -- caused by landing on another player's foot -- and not a stress reaction, and that the Sixers are supportive of a conservative timetable for his recovery.
Simmons told reporters Monday that he weighed 217 pounds before Philadelphia drafted him in June and has gained 33 pounds over the three-plus months since, sparking speculation that the weight increase was a factor.
But sources told ESPN that Simmons weighed in at 240 pounds in July shortly after he was drafted when he played for Philadelphia's summer league squad in Las Vegas -- where he was lauded for his passing eye and movement -- and reported to training camp earlier this week at 244 pounds.
Two sources with knowledge of doctor examinations of Simmons said Saturday that the injury is an acute fracture of the fifth metatarsal bone caused when he landed on the foot of teammate Shawn Long during a scrimmage Friday at Stockton University in New Jersey. The Sixers, wracked by injuries to several top draftees in recent years, were initially hopeful Simmons merely rolled his ankle.
In addition to the team physicians who have examined the 20-year-old, sources said, Simmons' advisers have consulted foot specialist Brian Donley of the Cleveland Clinic as the parties continue to pursue a firm recovery timetable.
Simmons' setback is the latest blow for a franchise that has been forced to wait for its first glimpse of numerous recent draftees.
Joel Embiid, selected No. 3 overall by the Sixers in 2014, has yet to play in the NBA because of multiple surgeries on his right foot. Nerlens Noel, drafted No. 6 in 2013, was forced to sit out his entire rookie season in 2013-14 with a torn ACL. No. 3 selection Jahlil Okafor, the No. 3 selection in 2015had his rookie campaign cut short after 53 games by season-ending surgery on his right knee.
In addition, Dario Saric played his first two pro seasons in Turkey after being selected by Orlando with the No. 12 overall pick in 2014 and being traded to Philadelphia on draft night. Saric signed with the Sixers in August and is in the midst of his first NBA training camp.
Simmons is the Sixers' first No. 1 overall pick since Allen Iverson in 1996.
The former LSU star won plaudits for his performance through the first week of training camp.
"I never played with somebody [like him]," Saric told Philadelphia reporters this week. "Everybody puts pressure on him, but I think he'll handle this."
ESPN Staff Writer Baxter Holmes contributed to this report.