It’s not time to panic, but it is time for the Celtics to take stock of their situation.

First, the bad news. They have lost three straight games at the TD Garden, and each time they have played poorly in the second half. The culprits have been defensive breakdowns in tandem with stretches of bad offense. Their record says Celtics, but they have played in spurts like the Nets.

In the case of Monday’s loss to the Mavericks, the stretch lasted longer than a standard basketball run. The result was a blown 12-point lead en route to an utterly lopsided 99-90 final score. (Recap.)

Specifically, the Celtics' transition defense was lacking, a function of shortcomings in both sides of the game. As Ray Allen said, “One side of the ball affects the other.”

Now, the not-so-bad news. The Celtics team that we’re seeing on the floor isn’t necessarily the same one that we’ll see in April. Everyone understands that and it’s not an excuse, but it is the reality.

If Kevin Garnett isn’t healthy come playoff time, this team is not winning a championship. If he is, the Celtics — despite their recent strugges — should be legitimate contenders.

Right now, the Celtics can’t be sure where they'll stand. Garnett’s health is obviously a huge glaring question mark, to say nothing of Paul Pierce, Rasheed Wallace or Marquis Daniels, who have all missed time recently.

All of that leads us to our three things we learned Monday:

WHO ARE THEY, REALLY?

The Celtics haven’t been whole the entire season. They have not played one game with their entire complement of rotation players, which makes it tough to establish an identity.

“I’m obviously hoping we can get everybody back by the All-Star break,” Doc Rivers said before the game. “That’s the plan right now. Then we can start building. We like our record, but we’re not building. We’re winning games but we’re not improving.”

The Celtics have been treading water since their Christmas Day win against the Magic. Since then, their record is a pedestrian 4-7 overall and just 1-3 at home.

“We’ve got to understand that we are a defensive team first, regardless of who we put on the floor,” Pierce said. “At the end of the night, regardless of whether we shoot the ball well or not, we are a defensive team. That’s something that is never going to change about this team.”

It’s one thing to say it — or even believe it, as Pierce and his teammates clearly do — but it’s another thing to execute their defensive system, something that has been sorely lacking of late. Against the Mavs, it wasn’t just Dirk Nowitzki torching them for 37 points (more on that in a bit). It was also turnovers that fed the Mavs transition game and led to layups and wide-open 3’s.

“Second half, I thought our defense was horrendous,” Rivers said. “I don’t think we got back at all on defense. They had numbers every single time. We had our guards crashing the glass trying to get offensive rebounds. Our bench complaining to the refs. They beat us down the floor and they scored.”

This is not how the Celtics play and it is not how they see themselves, but it is who they are right now. That’s not injuries. It’s not a lack of depth and it’s not even really about Garnett.

That’s focus and execution, and the Celtics haven’t put together a complete game for 48 minutes against a quality team since Christmas. 

SHEED’S FOUL TROUBLE COST HIS TEAM

With a little more than five minutes left in the third quarter, Wallace picked up his fourth foul when he was called for an offensive foul. That sent him to the bench and allowed Nowitzki an opportunity to go off.

Without Garnett, the Celtics had no one else who could guard Nowitzki. Brian Scalabrine tried. Glen Davis got a shot, but he also was powerless to stop Nowitzki's game.

Wallace was typically diplomatic about the calls.

“They don’t like tough defense on them,” he said. “Of course I get a whole lot of [expletive] calls. That’s how the story goes. I ain’t worried about it. We’ll see them again. Honestly, I can’t even remember which one the fourth foul was. There was so many bogus jones, but I ain’t worried about it. Like I said, we’ll see them again. We’ll go down there in a month or so. There’ll be retribution.”

We’ll have to wait until March 20 to see if Wallace can live up to his words, but in the interim we can marvel at the game Nowitzki put together.

The Celtics held him to 15 points in the first half, and yes, that is the correct verb. “He had 15 in the first half, but they were a tough 15,” Rivers said.

Nowitzki is in the midst of his ninth straight season in which he will average better than 20 points and eight rebounds per game. Because of the way his career has turned out, and because of the way the Mavs blew their chances during his two best seasons, he may never get the credit he deserves.

“I don’t know if there’s ever been a player like Dirk,” Rivers said. “They run plays for him like we run for Ray.”

Nowitzki is a 7-footer who shoots like a guard, which makes him something of a prototype for the “Stretch 4” that has become so much in vogue this era. But he is so good that he even breaks the mold that he helped create.

It would have been interesting to see what Wallace could have done during that crucial stretch, or how the outcome of the game might have been different had he stayed on the floor. But we’ll never know.

PERK’S GAME CONTINUES TO EVOLVE

In the midst of the darkness, allow a little light. Kendrick Perkins put together a monster game offensively, going 7-for-10 from the floor and finishing with 14 points and 12 rebounds.

Perkins showed off every low-post move in his repertoire. He even showed off moves we didn’t know he had. With the master of the low post, Kevin McHale, in attendance broadcasting the game for TNT, it bears remembering that Perkins was drafted one spot after the immortal Ndudi Ebi was snatched up by McHale’s Timberwolves.

To say that Perkins has come a long way would be a massive understatement. Consider the fate of Ebi, who was a massive high school talent who wound up playing a grand total of 86 minutes in 19 NBA games over two seasons. Perkins played even less in his rookie season than Ebi did, but he has steadily improved to the point where it’s not unreasonable to suggest him as a legit All-Star candidate.

He got there through nothing but hard work and determination. In a season in which very little has gone according to plan, Perkins’ development has been a welcome, and unexpected, bonus for the Celtics.