LeBron's incredible gesture for injured Hayward

NBA superstar LeBron James has been praised for a brilliant act of sportsmanship after opponent Gordon Hayward suffered a horrific leg injury.

The Boston Celtics entered the season on a high but it took less than six minutes for them to crash when their All-Star recruit Hayward dislocated his ankle and fractured his leg in a sickening fall.

The NBA guaranteed plenty of drama and plot lines when they pitted the Celtics against rivals Cleveland in Tuesday's opening game of the season and it ended up being a thriller.

The Cavaliers staved off the Celtics 102-99, but it began in nightmare fashion when Hayward climbed high for an alley-oop pass from the Celtics' other off-season All-Star recruit, Kyrie Irving, midway through the first quarter.

Hayward collided in the air with LeBron James and crashed to the floor on his left leg in a horrifying way.

A hush fell over Cleveland's home arena as players grimaced, including the Celtics' Australian big man Aron Baynes, as Hayward winced on the floor and team doctors set the foot back in place.

LeBron looked visibly distraught after colliding with Hayward in the air, and went over to check on his opponent before he was carted off the court, shaking his hand and expressing his sympathies.

"That's great sportsmanship," commentators said. "That's the beauty of sport."

LeBron and former Celtics player Isaiah Thomas also went into the Boston locker room after the first quarter to check on their stricken opponent.

"My thoughts and prayers go out to Gordon," James said after the game.

"It was a pretty gruesome injury."

There was plenty of bad blood entering the game.

James and Irving had been teammates on the championship-winning Cavaliers, but Irving fled to the Celtics in the off-season to partner with Hayward who left the Utah Jazz and signed a four-year, $128 million ($A163 million) contract.

The injury will almost certainly end Hayward's season.

The Celtics led 12-9 when Hayward suffered the injury but appeared shell-shocked after seeing him taken from the court on a stretcher and the Cavaliers went on to take a 54-38 half-time lead.

The Celtics battled back to take the lead with one minute to go, but a James lay-up and a Kevin Love three-pointer gave the Cavs the lead and a final three-point attempt by Irving on the buzzer to force overtime missed.

Baynes played with aggression in his first game with the Celtics, challenging James at the rim, and contributed six points, five rebounds and one block in 19 minutes.

James fell just short of a triple double with 29 points, 16 rebounds and nine assists while Irving had 22 points and 10 assists.

with AAP