Kyrie Irving said Gordon Hayward's sickening injury on debut for the Boston Celtics is one of the worst he has seen as Cleveland Cavaliers star LeBron James sent his well-wishes.

Hayward, 27, fractured his left ankle early in Boston's NBA season-opening 102-99 loss to the Cavs on Tuesday.

The All-Star was making his bow for the Celtics after arriving from the Utah Jazz amid much fanfare but it ended it agony after coming down on his ankle as he jumped in an effort to catch a pass during the first quarter.

Celtics and Cavs players turned away in horror as Hayward grimaced in pain at Quicken Loans Arena, and fellow Celtics debutant Irving – facing his former team – was immediately reminded of Paul George's gruesome leg injury for Team USA in 2014.

"It's tough. I've seen a few injuries in my career and I've had a few," said Irving, who posted 22 points and 10 assists in Cleveland.

"Probably two of the worst ones I've been in the game watching - one was [Paul George] at USA and this one tonight. It's not a great sight to see but you gotta pick yourself up. Just keep on fighting, man.

"We understand that [Hayward], God-willing, will be fine, and we're going to continue to pray for him and be there for him as best we can."

Celtics head coach Brad Stevens said: "He's put in a lot of great work. And I thought he had his most comfortable week as far as feeling like he was going to play really well. But now we'll hopefully get a full recovery, right? And so it's a tough deal, but I guess that's part of it: the risk of injury. I really feel for him."

Four-time MVP and three-time champion James, who had 29 points, 16 rebounds and nine assists, added: "I've seen a couple of injuries like that in my lifetime. I've seen Paul George when it happened to Paul. I was watching the game with Shaun Livingston when it happened when he was with the Los Angeles Clippers. I was watching NCAA basketball when Kevin Ware was at Louisville.

"Those are the injuries that you never seen coming and you never want to happen, no matter who it is, no matter what the stature, no matter how much competitive nature that you have. It's just very unfortunate."