This time we mean it. This is it. This is the last waltz for Paul Pierce and his Celtics.

There have been many times over the past four seasons when it appeared the window was closing for this group of Celtics to make one final championship run.

I was standing in the locker room after the heartbreaking, bone-jarring loss in Game 7 of the 2010 NBA finals in Los Angeles. That’s when Pierce, Kevin Garnett and Ray Allen began getting asked about next year. There was uncertainty until Danny Ainge, Doc Rivers and Celtics ownership stepped up for one more year.

It was the same script in 2011, when they lost to the Heat in five games in the Eastern Conference semis.

Then, after appearing ready to eliminate the Heat in Boston last spring after erasing a 2-0 series deficit, the Celtics went on to blow Game 6 at home and lose Game 7 in Miami. Those questions of ‘Was this the last ride?’ came up again.

Ray Allen became Benedict Arnold and left for Miami. But Garnett was brought back for three years at $36 million. Paul Pierce is signed through 2014.

Like every Celtics fan, I kept wondering how long this group could keep it up. Jeff Green, Jason Terry and Courtney Lee, along with Avery Bradley and Jared Sullinger were supposed to be the [mostly] young legs to re-energize the Celtics.

But something is becoming very clear. The C's don’t have any gas left in the tank. And now it appears that window is closing mid-season.

Just when the Celtics seemed to have withstood the December blahs by running off six straight wins -- improving to 20-17 in the process -- they hit a pothole that has sent them off into the woods crashing head-on into a oak tree.

And it’s their captain who is trying to pick up the pieces and put them all back together.

On Thursday, he had the chance to tie the game against the arch-rival Knicks with the TD Garden crowd begging and pleading for one more chance to scream.

But Jason Kidd pressured him along the sideline and Pierce lost the ball out of bounds. Game over.

"Just down the stretch we had poor execution, simple and plain," Pierce said. "You know, when you need buckets, when the game is tight, you can't turn the ball over. You know, and that’s what we did down the stretch. Our defense was good enough to win. We held the Knicks who score a lot of points, to 89, 90, whatever score it was. We gave ourselves a good chance to win. But it was just our execution down the stretch was very poor.

"Like I said, it was just poor execution for the most part. If we get good shots and wide-open layups and a lot of the shots that are not falling, maybe you can say we are getting tight or something. But when it's just poor execution, it’s poor execution, on the turnovers. We've got to set screens, we’ve got to do things harder. We can’t turn the ball over, especially late in the game."

Pierce spoke like a man who wants to believe there’s another second-half run left in this team but can’t convince himself.

Just before the game, Pierce found out that he was not going to be selected to the Eastern Conference All-Star squad for the first time since 2007, a year before the "Big Three" era. That’s when he missed half the season with a stress reaction in his foot.

The last time a healthy Pierce didn’t make the All-Star team was 2001, his third season in the NBA.

But worst of all, he is the captain of a ship that is taking on water very, very quickly.

After Thursday’s 89-86 loss to the Knicks, a loss that dropped the Celtics to 20-22, Doc Rivers was informed that Pierce didn’t make the team.

"So, Paul didn’t make the All-Star team. Well, our record had a lot to do with it," Rivers said. "He played well enough to make it. But listen, we’re two games under .500, and we already have two guys on the All-Star team."

Rivers went on to suggest that coaching by-laws also factored in Pierce being left off the team. But that’s hardly a pressing concern right now.

The Celtics lost their fifth straight Thursday night after winning six in a row. Going into Thursday’s action, Pierce and Garnett were the only Celtics to start in every game this season. Pierce leads the Celtics in scoring at 18.9 points per game.

He’s doing everything he can, even in the midst of a slump that even proven veterans would have a hard time ignoring.

He had 22 points Thursday but was 6-for-15 in the process. He was 3-for-15 in 37 minutes in the low point of the season, a 95-90 loss Tuesday night in Cleveland.

Since going 10-for-18 in an uplifting 102-96 win against the Knicks in New York on Jan. 7, Pierce has gone into a deep freeze. In the ensuing eight games, he failed to reach double figures in field goals. He has made just 41 field goals while attempting 113. That’s an ice-cold 36.2 percent.

He’s had stretches like that before, but Allen or Garnett had always been there to pick up the slack. Not now. And when Rajon Rondo picks up the slack, an odd thing happens. Including Thursday, the Celtics are 2-5 when Rondo scores at least 20 points.

Eastern Conference doormats (Detroit and Cleveland back-to-back on the road) have proven to be opportunistic vultures preying on the weak.

Rivers has done all he could, including pleading publicly for Ainge to make changes to shake up his roster.

It’s time for the captain to step up, on and off the court and make the same plea to his teammates.

"I really liked the effort, I liked the way we competed," Pierce said after the loss to the Knicks. "But we have got to do it night in and night out. Our offense definitely has to get better. There’s nights where our offense is really letting us down when we put this type of effort on defense in."

The Celtics fell behind 80-70 with eight minutes left in the game. They stepped on the pedal and drew to within two points but couldn’t pass the Knicks in the fast lane. That’s a formula for a head-on collision, or at the very least, running off the road and into a ditch.

It was the same against the lowly Hornets eight days earlier. It was the same against the woeful Cavaliers two nights earlier in Cleveland.

The sad truth is finally settling in for Celtics fans. Paul Pierce and the Celtics are finally out of gas. 

Now to the Trags Bag:

We ask: With Patriots season over, what will you be turning your interest to and why? Bruins, Celtics or Red Sox?

@McMahon6484 Trags, only one sport is as exciting as football is Bruins

@matt617beantown Bruins for now because Celtics kick into gear before playoffs and Red Sox too far away still

@BostonGal4Ever Trags, Definitely Celtics. B-ball has always been 2nd in line after football.

@tommyb_31 Trags, Bruins, at least they are out there giving 100 percent. Celts look like they are out there for the paycheck and its still early for the Sox.

Jeff Lowenstein (via Facebook): Celtics all the way. I want to see how Danny and Doc get out of this mess. And I'll wait until the playoffs to get excited about the Bruins. Think of it as my own "lockout" of spending any money going into Jacobs' pockets until May.