Embiid – enjoying an MVP-calibre season – will undergo an MRI after appearing to hyperextend his knee in a scary fall following a dunk in the third quarter of Friday's 127-101 win over the lowly Wizards.
Back in the line-up after sitting out Sunday's All-Star Game and Thursday's victory against the Chicago Bulls due to coronavirus contact tracing, Embiid posted 23 points on eight-of-11 shooting to go with seven rebounds, three assists and two blocks prior to exiting in pain.
Embiid stayed down for a couple of minutes before eventually hobbling back to the locker room at Capital One Arena in Washington.
As the Eastern Conference-leading 76ers sweat on Embiid's fitness, given his importance to their championship aspirations, Rivers told reporters: "I thought there was a little contact when he went up to dunk the ball.
"Thought he fell with his balance off. I'm not going to speculate, tomorrow [Saturday] we'll know, we'll have all the information for you. Hoping for the best.
"I did talk to him, he was in the locker room. He's in pretty good spirits so let's just hope for the best."
Heading into the Washington matchup, Embiid was averaging a career-high 30.2 points per game – second only to Wizards star Bradley Beal (32.5) in the NBA.
Embiid is fourth in the league points, assists and rebounds per game (45.0), behind Denver Nuggets star Nikola Jokic (46.8), reigning MVP Giannis Antetokounmpo (46.0) of the Milwaukee Bucks and Dallas Mavericks guard Luka Doncic (46.0).
Prior to Friday's trip to Washington, the 26-year-old Embiid has also been averaging career highs in field-goal percentage (52.1) and three-point percentage (41.6).
"We don't do anything right now because we don't have any information, so we're just gonna wait," Rivers said. "I thought our guys took care of the game which is great, and then tomorrow we'll get some more information and then we'll go from there.
"Listen, it's a long season, so at the end of the day, hopefully, it's not something where we lose him, obviously. But our guys will be good, we'll be ready."
The 76ers (26-12) have won four consecutive games to top the east ahead of the star-studded Brooklyn Nets (25-13).