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.

Police say they were able to catch the guy shortly after the throw ... identifying him as 22-year-old Justin Arnold from Everett, Mass., and hitting him with a misdemeanor charge.

The guy is now facing up to one month in jail for the incident.

For the Celtics' part, Stevens issued an apology after the game over it all, saying, "I'm sure the Celtics have already addressed this as an organization, but if not, I apologize to the Spurs for what happened out of the stands, and that can being thrown."

Stevens continued, "I told [Spurs coach Greg Popovich] after the game I'm really sorry that they had to experience that ... I hope that person's not allowed in an NBA arena again."

As for the game, the Celtics ended up losing after the wild sequence of events ... falling 129-114.