Way back in mid-December, the Celtics and Knicks played a classic, possibly the game of the year in the NBA regular season. It featured everything from in-game comebacks to gritty performances and late-game heroics. At the time the talk was of the up-and-coming Knicks and whether they could make an honest rivalry against the proud, veteran Celtics.

Things changed dramatically for both teams around the treading deadline when the Knicks cashed in a good chunk of their future for Carmelo Anthony and Chauncey Billups, while the Celtics dealt Kendrick Perkins and jettisoned four other players. Of the 18 players who saw time in that early winter classic, only 10 are still with their respective teams, five on each side.

The way Doc Rivers sees it, with all the roster turnover the Celtics and Knicks have played just one meaningful game this season. That was back in March when the Celtics shook off a terrible first half – prompting Rivers to call his team “soft” at halftime – and rebounded for one of their more satisfying victories of the second half of the season.

Yet for both teams this series isn’t really about what the other team does, so much as what they want to do themselves. The Celtics want to establish – or reestablish – their defensive identity, while the Knicks will bank on their offensive firepower making this is a classic matchup of contrasting styles. In a sense both teams are still trying to figure themselves out.

“We need to play a style of play that we’re used to playing,” Kevin Garnett said. 

Over the last six weeks that familiar style has been inconsistent at best and nonexistent at worst. While the Celtics have been hit or miss from game to game, the Knicks have run hot and cold with a late seven-game winning streak following a six-game losing skid.

The Knicks are calling themselves dangerous, and they are. If any underdog team is capable of stealing a game or two based on sheer individual talent, it’s them. The Celtics will have to be on guard, especially in the first two games at home.

Last season they were able to ease themselves into the postseason with a first round series against the Heat that flew way below the radar screen. No such luck in this one. The attention will be focused on them and we will soon find out whether their late-season struggles were an accepted part of the way they do business or a harbinger of an early demise.

Here’s how the matchups break down:

Rajon Rondo vs. Chauncey Billups

Way back in 2008 the question in the Eastern Conference finals was whether Rondo would get outclassed and outmuscled by the reigning Eastern Conference All-Star. Now it’s whether Billups can keep up with Rondo’s superior speed. Of course, Rondo has to use his speed for him to be effective.

If the early 2010 version of Rondo shows up, this is a clear advantage for the Celtics. If it’s the passive-aggressive Rondo of the last month and half, then they will have their work cut out for them. Even if Rondo plays up to his abilities, Billups remains a dangerous 3-point shooter and smart, savvy veteran. There are few players in the league the Celtics respect more than him.

“Leadership. Experience. Chauncey is that one player that can take a game over,” Garnett said. “Obviously they have two offensive juggernauts that can put up a lot of points individually. He understands pace. He understands flow. He understands what it takes to win.”  

If the Celtics are going to prosper in this series, Rondo has to win this matchup.

Ray Allen vs. Landry Fields

The Knicks rookie has had his moments, but he has noticeably tailed off in the second half of the season. Whether that is due to the trade that shook up the roster or just a standard rookie slowing down is open for debate.

The Celtics need to get Allen on track. The last time he attempted more than 11 shots in a game was in their meeting in New York a month ago. This should be a clear advantage for the Celtics as long as they play through Allen’s strengths. Expect to see a lot of Toney Douglas at this spot for New York.

Paul Pierce vs. Carmelo Anthony

Welcome to the marquee matchup. Ever since Anthony arrived the question for the Knicks has been whether they should play through Anthony or continue to work through Stoudemire. Rivers, for one, thinks that’s too simplistic.

“They do run more isolations, but they haven’t changed their offense that much,” Rivers said. “It’s the same movement but they do have another player they can feature. They run the same stuff for Amar’e coming up to the elbows, through movement. It makes them difficult because they have more than one guy now.”

Over the last four years, Pierce has squared off with LeBron James, Tayshaun Prince, Hedo Turkoglu and Ron Artest. He has always prided himself on playing both ends of the floor in those individual contests and that’s about as much of his plan for Anthony as he felt like sharing.

“Offensively I want to make him work,” Pierce said. “At both ends of the court I just want to make him work.” 

Both players are obviously important for their respective teams, but the difference here is that while the Knicks need Anthony to play like a superstar throughout this series if they are going to have a chance to pull the upset, the Celtics can expect help from other corners beyond Pierce. Still, this is the kind of challenge that he lives for.

Kevin Garnett vs. Amar’e Stoudemire

While Pierce and Anthony will capture their share of the headlines and Rondo and Billups will be the barometer matchup, Garnett and Stoudemire is one of the hoops connoisseurs. Stoudemire is one of the premier scoring big men in the game, while Garnett is the standard-bearer defensively at his position.

“What makes Amar’e so good is he’s explosive,” Garnett said. “He’s very aggressive. He can score in a lot of different ways. They run a lot of different sets for him. Any time he has the ball he’s always what we call live. He’s a forceful presence.” 

Stoudemire will score. He dropped 66 points in their first two games, including a scintillating 39-point effort in the December game. Garnett is a savvy enough defender to know that he won’t win the 1-on-1 matchup every time, but the key will be whether he can slow Stoudemire down in crunch time.

It wouldn’t be surprising if the Celtics started with Jermaine O’Neal on Stoudemire, but Rivers said Thursday that Garnett will play the majority of his minutes against him.

CENTERS

This isn’t really an X-factor so much as it’s an unknown factor. Will Shaquille O’Neal play meaningful minutes? Will he play at all? How will Rivers use Jermaine O’Neal, Nenad Krstic and Glen Davis?

The Knicks, meanwhile, essentially have no center. Mike D’Antoni said he will start Turiaf and the Knicks will need every bit of his toughness and rebounding. But look for them to essentially use Stoudemire as their “center,” and the Celtics to counter with Garnett and a smaller lineup featuring Pierce and Jeff Green up front. The Celtics can also utilize their most consistent lineup this season with Davis playing alongside the other four starters.

Any time Shaq can play there are obvious benefits for the Celtics offensively, but this is one series they can afford to pick their spots. Provided he’s capable of playing.

THE BENCHES

Rivers likes to say there isn’t a second unit in the postseason. “There’s a first and a half unit because it’s usually a combination of both,” he said.

As long as Delonte West is able to go with his ankle injury, this should be an advantage for the Celtics. With West, Green and Glen Davis, Rivers has three versatile players that he can plug into a number of different roles and positions. He can also go big when he wants to -- even without Shaq -- against the smaller Knicks. Douglas has been the one consistent reserve for D’Antoni all season and he often uses shorter rotations even in the regular season.

On paper this is a good matchup all around for the Celtics. Even at New York’s strongest positions – the two forward spots – the Celtics can counter with All-Stars of their own. The Celtics are deeper, bigger and more versatile. But they have to play up to their abilities and if they don’t, the Knicks have more than enough firepower to give them problems.