Hayward's left leg bent awkwardly after he went up for an alley-oop pass, collided with LeBron James and collapsed under the basket with 6 minutes, 45 seconds left in the first quarter of the season opener.
Players initially kept their distance as doctors tended to Hayward. Celtics teammates and Cleveland Cavaliers players, including James, then checked on Hayward as he was being stretchered from the court.
After the game -- a 102-99 Celtics loss -- Boston coach Brad Stevens said Hayward dislocated his ankle and fractured his tibia. The team announced the injury as a fractured left ankle. Hayward was set to fly back with the Celtics to Boston on Tuesday night and go directly to New England Baptist Hospital after landing.
ESPN's Michael Wilbon reported after the game that Hayward would have surgery Wednesday. But Hayward's agent, Mark Bartelstein, told ESPN's Adrian Wojnarowski that Hayward would evaluate a course of action Wednesday after undergoing MRI and CT exams in the morning.
Hayward signed a four-year, $128 million contract with the Celtics in July, making him the first of two All-Stars they acquired this summer. Boston later picked up Kyrie Irving in a trade with the Cavs. Hayward's arrival in Boston reunited him with his former Butler coach, Stevens.
"You hurt for him. 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," Stevens said Tuesday night. "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."
Hayward's left ankle turned at such an odd angle that teammates stood in shock as he motioned for help from the sideline.
Cavaliers guard Dwyane Wade knelt with his hand on his head nearby, as team doctors worked on Hayward. Shellshocked Celtics players huddled, with Irving burying his head in the chests of teammates Marcus Smart and Jayson Tatum.
Fans gave Hayward a standing ovation as he was taken from the court with his entire left leg immobilized in an air cast. Sources told ESPN that Hayward's ankle was reset on the floor, which reduced his pain and stabilized him.
He was immediately taken to the Cavaliers' locker room for more evaluation.
Numerous doctors and Cavaliers and Celtics officials were with Hayward in the Cleveland locker room, where he received X-rays. Many were ashen as they left, including Cavaliers general manager Koby Altman.
Isaiah Thomas was in the locker room with Hayward for an extended period before returning to the Cleveland bench. Thomas recruited Hayward to Boston this summer before being traded to the Cavaliers.
Cavaliers owner Dan Gilbert, who recruited Hayward to Cleveland as a free agent in 2014, also visited him in the locker room.
The Celtics trailed by 16 at the half but rallied in the third quarter.
As far as what the coach told his team at halftime about focusing on basketball after the injury, Stevens said, "I don't have any magic words for that. You're all hurting for him. I'm not going to try to take the human element out of it."
Players across the league immediately reacted on social media to the injury, including Paul George, who broke his leg in a gruesome injury in 2014.
God bless you bro @gordonhayward ! 🙏🏾 help him thru this god!— Paul George (@Yg_Trece) October 18, 2017
"It was terrifying," said Golden State Warriors coach Steve Kerr, whose team was preparing to play its season opener against the Houston Rockets. "The whole coaches' office was just devastated watching it. It just shows the fragile nature of what we do."
The Celtics play their home opener against the Milwaukee Bucks on Wednesday night.
Information from ESPN's Brian Windhorst was used in this report.