The team said Okafor has a small tear of the meniscus.
"It will take six weeks before he's able to get going practicing again and get back on the court, and so based on the [timetable], that is correct [that he'll miss the rest of the season]," Sixers coach Brett Brown said.
Okafor has not played since Feb. 28, when he collided with teammate Isaiah Canaan late in a loss to the Orlando Magic. Okafor missed four games because of a shin contusion, then missed Wednesday's 118-104 loss to the Houston Rockets because of knee soreness.
The 20-year-old Okafor missed two games in December with right knee soreness as well. He is averaging 17.5 points and 7.0 rebounds in 53 games this season.
Jerami Grant moved into the starting lineup with Noel out.
Information from The Associated Press was used in this report.