Hayward's "foot looked to be at a 90 degree angle from his leg," as the Washington Post's Tim Bontemps described it, and he was stretchered off the floor.
After the game, head coach Brad Stevens told reporters Hayward had suffered a fractured tibia and a dislocated ankle.
ESPN's Brian Windhorst reported LeBron James went back to the locker room to check on Hayward at the end of the first quarter. Isaiah Thomas, who recruited Hayward to come to the Celtics during the summer, also joined him in the locker room, according to Windhorst.
Losing Hayward for an extended stretch would be devastating after the team signed him to a four-year, $128 million deal this offseason.
The 27-year-old joined the Celtics after the best year of his career with the Utah Jazz, where he earned his first All-Star berth while setting new personal bests with 21.9 points and 5.4 assists per game. He also showcased impressive efficiency with a 53.6 effective shooting percentage.
With Hayward sidelined, the Celtics figure to turn to their young pair of No. 3 overall picks, Jaylen Brown and rookie Jayson Tatum, to fill the void.