He went down less than just six minutes of the first quarter after colliding with LeBron James and Jae Crowder in the air.
And it caused him to land awkwardly on his left ankle and twisting with Hayward immediately aware of the severity of the injury.
He was attended to by medical staff from both teams on the court before being stretchered off to the changing rooms and was later flown to Boston.
It has now been confirmed Hayward will undergo surgery and no timescale has yet been set for his return.
His injury had a clear impact on both former and current players who flooded social media with messages of support along with thousands of well-wishes from NBA fans.
And while Hayward couldn’t be there in person for the Celtics’ home opener against the Bucks this evening, he made sure to wish his team good luck ahead of the match.
As the players were introduced to the crowd, they were prompted to look up to the screens for a special message from the stricken Celtics star.
“Hello everybody I just wanted to thank everybody that has sent their thoughts and prayers,” Hayward said.
“I’m going to be all right.
“It’s hurting me that I can’t be there for the home opener.
“I want nothing more than to be with my team-mates and walk out on that floor tonight but I’ll be supporting you guys from here and wishing you the best of luck.
“Kill it tonight.”