Less than six minutes into his first game as a member of the Boston Celtics, All-Star small forward Gordon Hayward landed awkwardly on an alley-oop attempt and fractured his left ankle, suffering a horrific injury that resulted in him being carried off the court on a stretcher.

With the Celtics holding a 10-9 lead over the Cleveland Cavaliers at Quicken Loans Arena, Hayward made a back-door cut from above the 3-point arc behind the defense of ex-Celtic Jae Crowder. As Hayward leapt to corral an alley-oop pass from ex-Cavs point guard Kyrie Irving, Cleveland star LeBron James rotated over to try to break up the play. Hayward got caught in the air between James and Crowder, and landed awkwardly and very, very hard on his left leg.

TNT announcer Kevin Harlan immediately recoiled at the play, saying “Oh my goodness,” followed quickly by “Hayward broke his leg.” The other players on the court and on the Cavaliers bench all immediately reacted in shock as they watched the 27-year-old Hayward writhe in pain in the lane, his left leg gnarled in a horrifyingly unnatural position reminiscent of the injury that Paul George suffered during a USA Basketball scrimmage in the summer of 2014.

Coaches and trainers raced over to his side. Fans at Quicken Loans Arena coalesced into a stunned murmur. Players from both teams knelt on the court.

“And that is how quickly a season can change,” Harlan said.

After spending the first seven seasons of his NBA career with the Utah Jazz, developing from a reedy swingman out of Butler into one of the best playmaking forwards in the league, Hayward signed a four-year, $128 million maximum-salaried contract to come to Boston and play for his old college coach, Brad Stevens. Celtics president of basketball operations Danny Ainge viewed Hayward as a vital piece of a retooling effort aimed at toppling LeBron and the Cavs — a versatile ball-handler, facilitator, scorer, spot-up shooter and defender who could play off the ball alongside fellow new arrival Kyrie Irving and incumbent big man Al Horford, and give the Celtics the kind of punch they haven’t had at small forward since Paul Pierce’s heyday.

Those best-laid plans appear to have gone by the wayside — for now, at least — less than half a quarter in Hayward’s first game in green.

Hayward left the court on a stretcher with his left leg immobilized, receiving well wishes from his teammates and opponents, and hearing supportive cheers from the crowd in Cleveland. Near the midway point of the second quarter, the Celtics offered an initial diagnosis: “Fractured ankle is just the earliest information on GH. MRI will likely reveal the full extent of the damage. Just have to hope for the best.”