Boston entered the NBA season on a high but it took less than six minutes for them to crash when Gordon Hayward, one of their star recruits, broke his ankle in a sickening incident.
Hayward climbed high to collect an alley-oop pass in the first quarter, collided with the Cleveland Cavaliers' LeBron James and fell awkwardly on his left leg in the season opener.
A hush fell over Cleveland's home arena and players from both teams, including James, grimaced as they realised Hayward's gruesome injury was likely to end his season.
Celtics players gathered in a huddle to regroup.
"You hurt for him," Celtics coach Brad Stevens said.
"It's a tough, tough deal, but I guess that's part of the risk of injury. I really feel for him."
Boston added former Utah Jazz forward Hayward and former Cavaliers' guard Kyrie Irving, both All-Stars, and Australian Aron Baynes in the off-season to challenge the Cavaliers in the Eastern Conference.
Queenslander Baynes was brought on the court soon after Hayward was taken away on a stretcher, and was involved in a physical battle with James and Cavs forward Jae Crowder.
Cleveland went on to win 102-99, with James scoring 29 points to go with 16 rebounds and nine assists.
"My thoughts and prayers go out to Gordon's family," James said. "Hope for a speedy recovery, a healthy recovery.
"Those are the injuries that you never see coming, 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."
