Harden was injured during the Sixers' loss at Washington on Wednesday, said the person who spoke to The Associated Press on condition of anonymity because the injury had not yet been revealed publicly.
Harden stumbled on a drive to the basket early in the game and remained on his back in pain before he got up and returned to the game. Harden still scored 24 points.
The Sixers are 4-5 heading into Friday's home game against the New York Knicks.
The Sixers have already been hampered this season playing games without NBA scoring champion Joel Embiid because of illness. Sixers coach Doc Rivers said Embiid has the flu and has not been around the team in recent days to avoid spreading it around the locker room. Embiid, who has averaged 27.2 points and 9.5 rebounds in six games, missed Monday night's game in Washington for the same reason.
The 33-year-old Harden averaged 22 points and 10 assists in the first nine games of the season. Harden, the 2018 MVP, had seemingly recovered from lingering tightness in his left hamstring last season that decimated his explosiveness and forced him into what he called ''tough times, lot of dark moments'' in his personal life during the recovery.
The Sixers play six of their next seven games at home, including what could still be an anticipated Nov. 22 game at the Wells Fargo Center that would seen Harden go against his old Brooklyn team and the player he was traded for, Ben Simmons.
In Harden's 2017-18 MVP season, he joined Michael Jordan as the only players to average at least 20 points, eight assists, five rebounds and 1.7 steals in a season. He led the league in scoring, 3-pointers and 50-point games with four and led the Rockets to an NBA-best 65 wins.
The seven-time All-NBA player was also named last year to the league's 75th anniversary team.
More AP NBA: https://apnews.com/hub/nba and https://twitter.com/AP-Sports