Hayward, in his debut with the Celtics, appeared to have jumped and landed on his leg at an angle. After falling, Hayward remained on the ground, and can be seen grimacing in pain, his foot pointing at an awkward angle.
The Celtics were playing in Cleveland, and the fall happened in clear view of the Cavaliers bench players, who could be seen cringing and looking away.
Both players and fans looked shocked as Hayward was carted off the court.
Hayward helped lead the Utah Jazz to one of their most successful seasons this decade. He left the Jazz last season, and was signed to the Celtics.
It is unclear whether or not Hayward will be out for the rest of the season.
Right now he is being treated at New England Baptist Hospital in Boston.
Warning: Tweets may contain graphic images of the injury