Gordon Hayward's Celtics debut came to a horrific early finish, breaking his lower left leg and dislocating his left ankle halfway through the first quarter of the NBA's season-opening game.

"Hayward has broken his leg, Hayward has broken his leg," TNT broadcaster Kevin Harlan repeated immediately upon Hayward collapsing.

The Celtics and Cavaliers' medical staffs both examined him and determined it was an ankle fracture, the Celtics said in a news release. Coach Brad Stevens then clarified that it was actually a dislocated ankle and fractured tibia, which is part of the lower leg, according to Tim Bontemps of the Washington Post. Hayward was carted off the court after being attended to by medical staff.

The Celtics were leading 12-9 when Hayward left at the 6:45 mark in the first. The Cavaliers went on a 20-7 run to finish the first quarter up 29-19.

The 27-year-old small forward is coming off the first All-Star season of his career, averaging 21.9 points, 5.4 rebounds and 3.5 assists a game in a breakout year. He signed with Boston this offseason to play for his college coach, Brad Stevens, after seven seasons with the Jazz.

CBS Sports will provide more info as it comes out.