Hayward will visit a specialist on Monday to determine if he will need to undergo surgery, according to Adrian Wojnarowski of ESPN:
Hayward had nine points on 4-of-9 shooting in 15 minutes prior to exiting the game.
The injury interrupts what has been a strong start to the season for the 10th-year veteran. He averaged 20.3 points on 56.4 percent shooting, including 44.4 percent from three-point range, while adding 7.9 rebounds and 4.6 assists through the first seven games.
Hayward is just four days removed from a historic performance. According to Marc D'Amico of Celtics.com, Hayward became the first player since Wilt Chamberlain in 1967 to shoot 16-of-16 on two-pointers during Tuesday's 119-113 victory over the Cleveland Cavaliers.
He matched a regular-season career high with 39 points in the contest.
It marks the second time in three seasons since Hayward signed a max four-year, $128 million contract with Boston that he has suffered a significant injury. He dislocated his left ankle and fractured his left tibia in the 2017-18 season opener.
This latest injury will be a big blow to the Celtics, who are off to an Eastern Conference-best 6-1 start. Hayward, Jayson Tatum (21.6 PPG) and Kemba Walker (24.3 PPG) all entered Saturday averaging 20-plus points per game this season.
With Hayward sidelined, sixth-year guard Marcus Smart is a candidate to move into the starting lineup.