"Hayward has broken his leg", legendary announcer Kevin Harlan uttered on the TNT broadcast.
Hayward left with 2 points and 1 rebound in just over 5 minutes played.
The injury drew the game to a halt as the players & crowd stood around stunned in silence.
The Celtics announced he has a fractured left ankle. Fans of course should class and gave Hayward a standing ovation after he departed the floor. Shell-shocked Celtics players huddled, with Kyrie Irving burying his head in the chests of teammates Marcus Smart and Jayson Tatum.
The injury occurred in front of the Cavs' bench, and the video footage of the players' reactions shows the severity of the situation.
The injury cast a pall on the opening game of the NBA's 2017-18 season. He was immediately tended to by medical personnel and was taken away on a stretcher.
Hayward came over to the Celtics from the Utah Jazz in the offseason as a free agent.