2:59 PM PT -- Justin Arnold has broken his silence following the arrest ... owning up to the drink throw.
“I’m sorry to the Boston Celtics organization and TD garden for what I did," Arnold told WEEI. "It was unacceptable and I take full responsibility. I’ll never stop loving Boston”.
12:52 PM PT -- A spokesperson for the TD Garden tells TMZ Sports Arnold is now banned FOR LIFE for allegedly tossing the can onto the court.
"This act is a serious violation of our guest code of conduct," the spokesperson said.
Police say they've arrested a man who allegedly heaved his drink onto the court during the Celtics vs. Spurs game Wednesday ... identifying the guy as a 22-year-old and hitting him with disturbing a public assembly charge.
Late in the third quarter of the Celtics' game at TD Garden ... Kemba Walker was ejected for arguing a non-call.
The Boston superstar was cracked on a screenplay ... and when no whistle came -- he went IN on the refs.
It didn't take long for the point guard to get tossed from the game ... and when Celts head coach Brad Stevens also got ejected for arguing the call -- a fan was so pissed, he lobbed his drink onto the court.
You can see in the video, the can -- allegedly full of hard seltzer -- BARELY missed San Antonio Spurs players ... and splashed all over the hardwood.