Ray Allen shoved the dagger into the Heat with a corner-pocket 3-pointer with 49 seconds left, and since he was the one who fed Allen the ball, Paul Pierce happened to have a bird’s-eye view of the game-clinching bucket.

He was, as he put it, “a witness.’’

The Celtics fended off a late surge by LeBron James and the Heat and Allen’s shot sealed the 88-80 season-opening win.

James’s “Witness’’ billboard — a Nike production — became a famous landmark during his time in Cleveland. It was taken down shortly after James left town.

“I’ve been a witness of it the last three years, and he’s one of the best players that I’ve ever played with,’’ Pierce said of Allen. “It’s an honor just to be able to step on the court with him night in and night out. You’ve got a guy that can take that kind of pressure off you, it’s an amazing feeling.

“I don’t know if you guys realize it, but Ray, he’s hit so many game winners for us and so many clutch shots for us, we have confidence to get him the ball in these situations. He delivers nine times out of 10.’’

At that point, Kevin Garnett chimed in, “And this guy hits big shots himself.’’

Pierce responded, “I like watching, too.’’

The setup was one of Pierce’s three on the night, and during the timeout before the play Celtics coach Doc Rivers made sure to stress the importance of the extra pass.

“We drew a play out of the timeout, and the only thing we said is if it’s not there, it won’t be there because they have to rotate, and if they rotate, if we make the next pass, the ball will find the open guy,’’ Rivers said. “And that was terrific. [Pierce] had a shot, but it would have been contested, and he made the extra pass.’’

Cavaliers owner Dan Gilbert got the bitter love letter out of his system this summer, and management did its best to pick up the pieces in the offseason. Now the Cavaliers will attempt to move on without James.

The Celtics get to see both sides of the biggest breakup in basketball history.

“It is a bit of irony,’’ Allen said. “Who set the schedule up?’’

Obviously, the season opener was hand-picked by the league, but the Cavaliers matchup was a bit of arbitrary scheduling cruelty. The Celtics opened the season with the Cavaliers the past two years, the opening chapters in a rivalry that became heated.

The city that hung billboards, composed tribute songs, and held rallies to persuade James to stay was devastated when he left.

“I’m sure they’ll be feeling those residual effects of having LeBron being gone,’’ Allen said.