The Sixers on Monday learned that Joel Embiid needs to enter the NBA's health and safety protocols, a source confirmed to NBC Sports Philadelphia.
The Athletic's Shams Charania first reported the news.
Monday night's game against the Knicks was a planned rest day for Embiid, the source said, but his availability moving forward is now uncertain.
The Sixers could get a starter back in Danny Green, who's listed as questionable after missing the previous three games with left hamstring tightness. Green put up jumpers after shootaround and appears close to a return.
Embiid became the fourth Sixer in the league's protocols, joining Tobias Harris, Isaiah Joe and Matisse Thybulle.
Sixers head coach Doc Rivers said last week Harris was doing "OK, but not great, honestly." The 29-year-old tweeted, "Every W is healing me" after tuning in for the team's sixth straight victory on Saturday night, a win over the Bulls in Chicago.
The Sixers have improbably managed to keep winning despite their health and safety-related absences, but losing Embiid is especially difficult in terms of on-court impact. The MVP runner-up, who posted 30 points and 15 rebounds against the Bulls, is massively influential on both sides of the ball.
Without him, Andre Drummond figures to start and Paul Reed should continue to play key minutes. Other center options include Georges Niang and rookie Charles Bassey.
To clear the NBA's protocols, a player must either wait a minimum of 10 days or return two consecutive negative PCR COVID-19 tests at least 24 hours apart.