Everything was much too easy for Philadelphia last night.

Unless you don’t have a Twitter account, you probably know that Lakers head coach Mike Brown got fired yesterday. The Lakers effectively pushed the panic button after an abysmal 1-4 start, one of the few bright spots for Celtics fans so far this the season. What’s interesting (and well-documented) is that while the national media pointed their spotlight at Brown’s unsuccessful attempts at running the Princeton offense (a set which, several intelligent writers pointed out, made up only a small percentage of Laker possessions), LA’s real problems lay on the defensive end. The Lakers had a defensive rating of 107.5 (or 107 points per 100 possessions) when Brown got fired, good for 25th in the NBA. So while Brown got shown the door for failing to build a high-powered scoring machine with four future Hall of Famers, the reason his team was losing was actually on the defensive end.

The Celtics can relate, unfortunately. Much has been made about the Celtics’ offensive woes, and there are certainly some very serious problems with the offense. The Celtics are 25th in points per 100 possessions (99.2). They are also 21st in points per game (96.0) despite being 10th in pace (94.3 possessions per game). And, as we’ve previously noted, there are also some pretty serious late game problems, as the offense tends to seize up like a stiff back.

But none of this is news, unfortunately. Boston was terrible on offense last year as well. So given the 2-3 start to the season, something other than the offense must be wrong, correct?

Sure enough, Boston’s defense has been painfully bad. Per Synergy Sports, the Celtics have given up 0.88 ppp overall through five games, good (or bad, if we are being technical) for 21st in the NBA. 25th in offense and 21st in defense is good enough to beat the Wizards without Wall and Nene, but it’s not good enough to beat many other teams.

So what’s going wrong? Why are the Celtics no longer an elite defensive team?

As is most often the case when a team is struggling, it’s impossible to pin down a single area in which they are struggling the most because everything is interconnected. But one area in which Boston has gone from excellent to mediocre so far this year is in post-up situations. Whereas last season the combination of Brandon Bass’ surprisingly stellar individual defense and Garnett’s typically excellent team defense made playing in the post against the Celtics a torture chamber, the Celtics have begun to allow teams to access the lower block much too easily. Here’s an example cherry picked from last night’s discouraging loss to Philadelphia:

I briefly lost my mind about this play on Twitter because, frankly, it’s disgusting for so many reasons. Let’s run them down:

• Thaddeus freakin’ Young runs a fast break, resulting in an easy basket in the post.
• He is so uncontested he manages to get into post position with 18 seconds left on the shot clock by dribbling the ball up the floor himself. Not by passing to his point guard and running to his spot, but by slowly dribbling the ball up the court hunched over like my dog when she has a bread crust and is trying to prevent me from taking it away.
• I don’t mean to pile on Jeff Green, but that is just the WORST kind of defense. He stands in  Young’s way at about the free throw line, but he isn’t REALLY a part of the play until Young has already hit the block and is halfway into his turn. At range that close, Green is screwed.

But even more disturbing than the post defense is the transition defense, which makes up 11.7% of all opponent opportunities. In 65 transition chances so far this year, opponents are (brace yourselves) averaging 1.65 ppp. That. Is. Awful. LeBron James, by way of comparison, averages 1.53 ppp in transition. So 11.7% of the time, Boston’s defense turns opponents into something SIGNIFICANTLY MORE PRODUCTIVE THAN THE BEST PLAYER IN THE WORLD. Perhaps you are starting to understand how Boston has started 2-3.

Spot-up jumpers remain the most often used opportunity against the Celtics, although it’s down from 20.7% to 17.6%, which would indicate that teams are finding it easier to get into the lane and aren’t having to settle for jumpers as much. Worse yet: last year, Boston held opponents to 35.1% on spot-up field goal attempts, which was #1 in the NBA. This year, opponents are shooting 41.9%.

Why are spot-up opportunities getting easier? The real answer is complicated, but the simplest answer could be painful: It’s possible that Boston’s older legs are having trouble contesting jumpers. Here’s an example:

This isn’t actually bad defense by the Celtics until the very last part of the play. Pierce does a decent job helping on a driving Jrue Holiday to prevent the speedy point guard from getting to the basket against Barbosa, and when Holiday kicks to the corner, Pierce doesn’t have a ton of ground to cover before he can have a hand in Holiday’s face, contesting the shot. But Pierce doesn’t have much quickness anymore, and he just doesn’t make it in time. Holiday gets a good look at the shot, and Pierce doesn’t get his hand up. Bang.

Here’s another example of a similar play:

This is a combination of slower defenders and the deep post problem I mentioned earlier. Garnett makes the right play, denying the pass to the point guard, but Hawes recognizes what Garnett is doing and slips toward the basket. Green tries to help, but again, Garnett isn’t fast enough to recover, and Hawes is simply too long-limbed to be stopped when he receives the ball that close to the basket.

There are other problems too, of course, but these are the ones that seemed to manifest themselves most prominently last night. The depressing thing is that, unless the Celtics are somewhat out of shape and are having trouble contesting shots because they haven’t worked themselves back into regular season conditioning yet, neither of these plays were anybody’s fault. It’s not Pierce’s fault that Barbosa needed help on Holiday. Garnett wasn’t really in the wrong in trying to deny the ball, and Jeff Green rotated over properly to help. In each case, it was just better offense.

There’s still a TON of time to improve. Perhaps the elder statesmen on the team just need some time to get themselves into shape. Perhaps the addition of Avery Bradley in December will help swing this team in the right direction, as it did last year. But as of right now, Boston’s defense is unacceptably bad, and there’s no real indication that any drastic change is likely to happen soon.

Follow Tom on Twitter: @Tom_NBA.