Mike Celizic

The playoffs are supposed to be a new season, where everybody who’s in it has a chance to win it. And in baseball, football and hockey, that’s pretty much the way it is.

But it doesn’t work that way in the NBA. As a rule, the best teams in the regular season are the best teams in the playoffs, and there’s no hot goalie or lights-out starter or brilliant game plan that’s going to change it. It’s why there aren’t a lot of Cinderella champions in the NBA record books.

It’s also why you don’t have to worry about the Celtics getting back to the NBA Finals, not this year and probably not any year soon. The team will say it’s still has its championship core from two years ago, and all it needs to do is jack up the intensity and watch out Orlando and Cleveland.

It was just in 2008, after all, that the Celts were the state-of-the art basketball team, possessors of the NBA’s best record as well as the game’s reigning Big Three superstars: Kevin Garnett, Paul Pierce and Ray Allen. They made the Eastern Conference playoffs more difficult than they needed to be, but ultimately they sent LeBron home early, then took out Kobe and the Lakers 4-2 in the Finals.

But 2008 might as well have been 100 years ago. That team won 62 games. This team will finish with 50 wins and a No. 4 seed in the East. That team had a healthy Garnett dominating the middle. This year’s has a brittle, aging Garnett. He missed a chunk of time in mid-season with a hyperextended knee, and the team has babied him down the stretch, afraid of another injury that could affect his postseason.

"We made our bed and now we’ve got to lay in it,’’ Garnett told reporters Tuesday after a loss to the Bulls. "We’ve just kind of got to make the best out of it. I think our goals were a lot higher than this, but we’ll take it.’’

Age is also taking its toll. Garnett will be 34 in May. Allen turns 35 in July. Pierce is the kid at 32.

Interactive NBA finals breakdown, analysis Ira Winderman breaks down the NBA finals between the Lakers and Celtics. NBCSports.com

As their ages increase, their numbers have decreased. Every one of the Big Three are scoring well below his career average. Pierce is at 18.3 points per game, 4.2 below his career average and the lowest total since his rookie season. Allen is at 16.3, also 4.2 below his career average and the lowest total since his rookie season. Garnett has fallen all the way to 14.3 points per game, 5.5 below his average and the lowest since his rookie season. Garnett’s rebounds are also the lowest since his rookie year.

What it all means is that the window of opportunity that so recently was flung wide open is all but closed. What looked like a green-and-white dynasty crumbled the following season, along with Garnett’s knee.

Garnett came back, but the magic was gone. The Celtics were still among the better teams, but it was no longer the best. They lost in the East semifinals to the rising Magic.

They had reached their level of greatness at the peak of the careers of their Big Three, and now it’s all running downhill. Only Rajon Rondo, their often spectacular point guard, is getting better, but he’s not a scoring machine who can carry the team through the playoffs.

The Celtics front office hasn’t filled in the gaps. Rasheed Wallace, their big offseason acquisition, isn’t what he used to be when he was helping the Pistons win a title. Wallace is only scoring 9 points per game and hitting 28.3 percent of his 3-pointers. And Nate Robinson, the midseason acquisition, hasn’t added anything to the mix.

So go ahead and fantasize that the Big Three will kick it into gear and carry the Celtics to playoff glory again. But don’t bet the house on it. Don’t even bet the porch.

Cleveland and Orlando are the powers in the East. The Hawks are a team on the rise. The Celtics are a team that used to be great and now is merely very good.