The team announced Thursday that Simmons suffered a subluxation, a partial tear, of the left knee cap.
He will be considering treatment options.
Simmons left the game in the third quarter as the 76ers held on to beat the Washington Wizards 107-98 in Lake Buena Vista, Florida.
Simmons landed awkwardly after grabbing an offensive rebound. At the next break in the action he walked gingerly to the locker room and did not return.
He was shown leaving the locker room in street clothes late in the fourth quarter.
"We are all coming to grips with the news we received," Coach Brett Brown said.
Brown said the news of Simmons' injury stings. He said he does not have a timeline for Simmons' return.
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ESPN contributed to this article