There is a common misconception about the Celtics that Kevin Garnett, Paul Pierce and Rajon Rondo are playing at their typical All-Star levels, and the team's role players are to blame for the subpar first half of the season.

In actuality, a team that was bound to see its championship window close quickly due to age may be ready to usher in a new era. Garnett and Pierce are playing well below the level they achieved even as recently as last season, and Rondo has fallen short in his quest to make the team his own.

The Celtics have played just over a half-season of losing basketball, with Thursday night’s 89-86 loss to the Knicks marking the first game of the second half. Here is a report card for a first half in which they posted a record of 20-21.

Kevin Garnett -- In Garnett's defense, the Celtics don't have enough size in the frontcourt, and he is playing out of his natural position of power forward the majority of the time he's on the floor. But statistically, this is setting up to be Garnett's worst season as a pro. Perhaps Garnett is pacing himself; his scoring average did improve last season from 14.4 before the All-Star break to 17.0 after. In that respect, Garnett's scoring average of 14.7 in the first half is no cause for alarm.

Now for the bad news: Garnett is averaging 7.0 rebounds per game, easily the lowest number he's posted since his rookie season in 1995-96. His assist average (2.1) is his lowest total since his rookie season. If you want to explain those numbers away by saying Garnett is also averaging the fewest minutes (29.8) per game of his career since his rookie season, here's the counter argument: Garnett's advanced stats are worse than the traditional ones.

Per 40 minutes, Garnett is averaging career lows in rebounds (9.4), assists (2.8) and true shooting percentage (54.0). His assist ratio is a career low (12.0) and his turnover ratio is a career high (9.9). Not good. With the minutes Garnett has logged over his career, his decline was inevitable. It's just that, at 36, it appears to be coming faster than many expected.

On the bright side, Garnett still appears to be the same emotional leader he's always been. During the Celtics' six-game winning streak just after Avery Bradley's return, Garnett had an incident in each of the first three victories. He had the flagrant foul on Tyler Hansbrough for which he was ejected, he was called a dirty player by the Hawks, and he got into a confrontation with Carmelo Anthony. Garnett still has the ability to spark the Celtics with his physical play. Unfortunately, he can't issue flagrant fouls and pick fights every night for an 82-game season.

Grade: B-

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Paul Pierce -- The departure of Ray Allen has hurt Pierce more than anyone else on the roster. Now that opposing teams no longer have to devote a man to shadowing Allen around the floor, Pierce is not finding as much space for himself, and he's starting to regress to a player similar to the one who played alongside Antoine Walker in the early 2000s.

Pierce's shooting percentage has declined each of the last three seasons, from 49.7 percent in 2010-11 to 44.3 percent last season to 42.3 percent this season. That's Pierce's lowest mark since the 2003-04 season. From a true shooting perspective, Pierce is also posting the lowest percentage since the 2003-04 season, and he's declined rapidly in the last three seasons.

That being said, Pierce still is averaging 18.9 points per game, down just a half-point from last season. In the Big Three era, Pierce has been almost certain to get his 20 a night, and that remains the case now. Starting in 2007-08, Pierce's average points-per-game totals by season are 19.6, 20.5, 18.3, 18.9, 19.4 and 18.9. These days, it just takes Pierce more shots to get his 20.

Grade: C

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Rajon Rondo -- When Ray Allen identified Rondo as one of the key factors that influenced his decision to accept a deal in Miami last summer, I expected Rondo to have an MVP-caliber season. That's what a superstar would do. He'd stew for three months preparing himself for the season, and then he'd spend the next six months proving that the Celtics are better off without Allen.

In evaluating Rondo's season, you also have to consider the fact that he called himself the best point guard in the NBA last offseason. He's also jumped at every opportunity to call the Celtics “his team” since September. So the grading scale certainly is skewed.

The problem with Rondo is he hasn't really improved at all this season. In his seventh season, it's clear he is not going to be a superstar. Can he be the point guard on a championship team? Yes, he already proved that. But he doesn't have the leadership traits to be the best player on a championship team. He mails it in too many nights, playing little to no defense.

Avery Bradley was exactly right after Kyrie Irving's 40-point night on Tuesday. Much of that came at Rondo's expense. Rondo also hasn't shown any evidence of maturation this season. He's served two one-game suspensions, showing no remorse in either instance.

Stat-wise, Rondo's basically the same player he's been since 2008-09. He had a better scoring year in 2009-10, and he had more assists per game last season, but overall, he's having one of his better years. He just hasn't been able to take it to the next level.

For those who believe there's a case for Rondo being the top point guard in the league, the advanced stats suggest otherwise. In terms of player efficiency rating, Rondo ranks 13th among point guards behind Chris Paul, Russell Westbrook, Tony Parker, Kyrie Irving, Kyle Lowry, Jose Calderon, Stephen Curry, Kemba Walker, Eric Bledsoe, Ramon Sessions, Jrue Holliday and Deron Williams -- in that order.

Grade: C+

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Avery Bradley -- It's difficult to evaluate Bradley given his small sample size, but his return certainly sparked an improvement in the team's perimeter defense. When he's healthy, Bradley is able to defend the opposing team's best perimeter player, giving Rondo an opportunity to move off the ball.

But therein lies the problem – Bradley's health. He has battled injuries ever since he emerged as a key player in the rotation last season. At 6-foot-2 and 180 pounds, he doesn't have the body mass to fight through screens in a pick-and-roll league on a nightly basis.

After the Celtics went 6-1 in Bradley's first seven games this season, he injured his ribs in a game against New Orleans. He hasn't been spending as much time chasing the opposing team's best player around the court since, as was evidenced by Rondo's defensive nightmares against Irving. Starting with the New Orleans game in which Bradley was injured, the Celtics have lost five games while their starting shooting guard has been on the mend.

Here's a promising stat for Celtics fans: Dating back to March 25 of last year when Bradley broke into the starting lineup, the Celtics are 20-9 in regular-season games when Bradley gets the starting nod.

Grade: A-

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Brandon Bass -- At 6-foot-8 and 250 pounds, Bass is one of the most frustrating pieces on a team that ranks last in the league in rebounding. In 26-plus minutes per night, Bass is averaging 5.0 rebounds to go along with his 7.7 points. Simply put, Bass is having a bad season. His shooting percentage dropped from 51.5 percent in his final season with Orlando in 2010-11 to 47.9 percent with the Celtics last season to 45.3 percent this season. He's averaging 4.8 fewer points per game this season compared to last, mostly because he hasn't been hitting his midrange jumper, which was automatic last season.

There's really no explanation for the decline. Bass is 27 years old, seemingly in his prime. Perhaps he's not happy with a reduction in minutes due to the Celtics' addition of Jeff Green.

Grade: D

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Jeff Green -- Speaking of frustrating … Green might be the most frustrating Celtics player to watch in person during the Big Three era. As the entire Boston fan base anxiously waits for Green to legitimize the trade that sent Kendrick Perkins to Oklahoma City, the Celtics forward passively jogs up and down the court, waiting for his teammates to kick it out to him in 3-point land.

Green was billed as a player who would make the Celtics younger and more athletic. Those athletic bursts have been few and far between, but they occur just enough to further frustrate fans. If Green is searching for a role, he would be best served by hitting the glass and using his 6-foot-9 frame to pull down more than the 3.1 rebounds per game he's averaging.

If Celtics fans were upset with the version of Green they got in 2010-11 post-Perkins trade, they're even more upset now. Green has been worse. His shooting percentage has plummeted from 48.5 to 42.1, and his scoring average has dropped from 9.8 to 9.4. But he is taking more 3-pointers!

Grade: D-

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Jason Terry -- When the Celtics signed Terry last summer, the idea was that he'd provide instant offense off the bench for a team that lacked bench scoring last season. Terry also was expected to be someone who could create his own shot when Rondo wasn't on the floor -- another area in which the Celtics struggled last year. When Rondo sat, the offense became stagnant.

The expectations for Terry have not been recognized by the player. He's shot his way to his lowest field goal percentage since the 2003-04 season. His scoring average has dropped from 15.1 per game last year in Dallas to 10.0 this year in Boston. Terry also has been a below-average defender, and he's been much better playing with Rondo than without him. However, the plan for Terry was to keep the offense moving when Rondo is not on the floor. In a related note, Terry is 35 years old.

Grade: D

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Jared Sullinger -- The Ohio State product has been the most (and arguably only) pleasant surprise for the Celtics this season. Sullinger knows how to play. He's averaging 6.2 points and 6.1 rebounds in 20 minutes of action. He never seems to mope around when his minutes are cut, and he never seems overwhelmed by the moment when he's put into critical situations. He has a low-post game few other rookies boast, and he seems to have a grasp of the Celtics' defensive system. From a talk-radio perspective, he is the one player fans bristle about tossing into trade conversations.

Grade: A

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Courtney Lee: Much like the Celtics in general, Lee has been up and down. Early in the season, he was criticized by Doc Rivers for being slow in picking up the defensive rotations. As recently as Jan. 5, he played less than nine minutes in a victory over Atlanta. That being said, he's probably better suited as a fourth guard, which is the role he's been playing since Avery Bradley returned. He almost seemed energized by Bradley's presence, and he's shown some ability to provide energy on the offensive end by cutting to the hoop and knocking down 3's.

Grade: C

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Leandro Barbosa, Jason Collins, Chris Wilcox, Fab Melo -- incomplete

Doc Rivers -- It's easy to give Rivers the benefit of the doubt considering this team is in Year 6 of the three-year Big Three plan. His players are old. There's nothing he can do about that fact.

On the other hand, he hasn't been able to ignite Green in two seasons with the team. If Green is as talented as Danny Ainge believes he is, you have to give Rivers some portion of the blame pie for failing to tap into that potential.

A key moment of the season for Rivers came Sunday after the embarrassing loss to Detroit. After months of calling his team "soft," Rivers threatened the jobs of some of his players. They came out two nights later and delivered a stinker against Cleveland, allowing 31 points in the first quarter in their fourth straight loss.

After the game, Rivers focused on the positive, spinning the loss as a potential step forward. Did he regret the public criticism of his team he offered two nights earlier? Maybe, maybe not. He can't rip his team publicly after every game without losing the entire roster in short order.

That being said, I expected the Celtics to show more life after Rivers' warning shot was fired. Even if players like Garnett, Pierce and Terry are getting old, why can't Green, Bass and Lee carry the team for a night? Are they unhappy with their roles? Embarrassed by the criticism from their coach? Time will tell, and it will help determine Rivers' ability to reach his players as the Celtics make their transition to a new era.

Grade: C