BOSTON – All that’s left for the Boston Celtics and their fans is frustration — frustration that Ray Allen accepted less money to play next season for the NBA champion Miami Heat, frustration that the Celtics’ offer of double Miami’s money and a no-trade clause wasn’t enough.

Actually, there’s another emotion, too — disappointment because of a growing belief that Allen never intended to stay.

Sources say Allen’s contract demand for the Celtics — $27 million over three years — would have tripled what he accepted from Miami.

Some would call it a proposal that was meant to be rejected. That theory is certainly believable.

Allen had a laundry list of reasons to leave.

He passed up a chance to sign for more money elsewhere two years ago, and was angered when he was dangled as trade bait for two straight winters. Allen was always the member of the Big Three who got along the least with Rajon Rondo, and it’s now believed the relationship became irreconcilable.

Beyond the scrape of personalities, no one was left out more than Allen when Rondo broke from script and went into so-called random mode.

Allen, who needs a structured offense because of his cuts and dependence on timing, was often ignored when his point guard morphed into Picasso at the end of a play. Allen’s ankle trouble over the last two months of the season only made it worse.

And then there was Allen’s move to a sixth-man role behind Avery Bradley.

Allen may come off the bench in Miami, too. The Celtics believe this was probably the least of Allen’s complaints.

But overall, that dissatisfaction was enough for Allen to walk away from a city where he and his family have made a significant impression.

That will ultimately be considered sad by all concerned, and already the nicknames are starting — Judas Shuttlesworth and Benedict Allen are two.

Outsiders don’t often understand the unforgiving passions that rule Boston sports, but all business is taken personally here.

Kendrick Perkins, Eddie House and even Glen Davis — players who didn’t leave on their own –have returned in other uniforms to touching video tributes on the Garden Jumbotron.

Johnny Damon became a Yankee for more money, and the Fenway crowd waved dollar bills during his first enemy at-bat. Bill Parcells became the Benedict Arnold of coaches when he joined the Jets, and after Curtis Martin followed for more money than the Patriots could afford, the running back wasn’t forgiven for a long time.

Allen didn’t leave for more money, but which group do you think he will fall into?

And yet despite this apparent desire to uproot his family, the Allens savored the difference they made in Boston.

Visit Boston’s Joslin Diabetes Center and walk through the door of the juvenile diabetes clinic, where young Walker Allen is a patient.

The walls are covered with Celtics artwork, the display centered by a Fathead-sized cutout of Ray Allen, who is on Joslin’s board of directors.

His Ray of Hope Foundation has equipped four Boston middle schools with computer labs, each worth $30,000. He recently funded two more school labs in Hartford. His wife, Shannon, has hosted a popular cooking show on Comcast.

And when the Garden crowd stood and cheered for the last three minutes of the Celtics’ Game 6 loss to Miami last month — Allen’s last home game as a Celtic, as it turned out — no one was more touched by the outpouring.

“I know I’m biased, but they’re the best fans I’ve ever played in front of or ever seen in my life,” he said that night.

“They understand the situation that is before us, and we understand it. And that was basically them sending us off, letting us know, hey, this is still well and alive. “

We need you guys to go down there and get a win for us. We all felt it on the bench. … It seemed like that three minutes lasted forever. It was special. I’ll talk about that forever, just knowing being down 20 and these people are still standing up, cheering us on.”

Allen will always have his bond with Kevin Garnett and Paul Pierce.

It remains to be seen whether another important tie — the one with that adoring crowd — survives for LeBron James’ new wing man.