As long as the Celtics employ Garnett, they will probably be good at defense.

Two weeks ago, I took an in-depth look at how Boston’s new pieces would affect the offense. A quick summary: The offense is likely to improve. It may not be a dramatic improvement, but it’s likely to be better than last year. Thank God.

But what about the defense? Are we going to continue to see one of the best defensive teams in the NBA in Boston, or will there be some slippage with a new rotation of players?

Earlier today, I posted this about Brandon Bass, based on stats I found for this article. I was shocked to find that Bass has, statistically, been one of the best defenders in the NBA. Some people on Twitter pointed to this finding as evidence that stats don’t tell the whole story. While I question their conclusions, defensive stats, at least individually, do remain somewhat of a mystery. It’s hard to quantify exactly how well a player defends using numbers, so we are forced to settle for a series of incomplete stats and the eye test (for example, I can see that Andre Iguodala and Tony Allen are two of the best perimeter defenders in the NBA).

So instead of looking at what each player brings individually, let’s take a look at where the Celtics played their best defense and how the new additions might help or hurt (all stats gathered from Synergy Sports).

The first and most important thing to be aware of is that the Celtics have been very good at defense over the years for two main reasons. First: Kevin Garnett is one of the best defenders in the league, and he has been for quite some time. Second: Doc Rivers has been running a very successful defensive system since 2008. Both Garnett and Rivers’ system will remain place in 2013. So there is a very good chance that the defense will continue to be excellent. But that doesn’t mean we can’t take a closer look at it, right? Guys? I lost you with the chart, didn’t I? Whatever, I’m going to keep going.

The best takeaway from the chart above is that the Celtics don’t rely on individually elite defenders to be a great defensive team, although it certainly doesn’t hurt that they have Kevin Garnett and Avery Bradley. How can you tell?

For starters, the Celtics were successful at keeping teams at an arms-length on offense. 20.7% of opponents’ shots were spot-up opportunities. This indicates that the Celtics kept teams away from the basket and in the range of inefficient jumpshots. But even the jumpshots were contested, as opponents only hit 35.1% from mid to long range. By way of comparison, the Sacramento Kings, who allowed more points per 100 possessions than any other team, forced opposing teams into taking spot-up opportunities 18.9% of the time, and opponents hit 39.2% of their shots. That may not seem like a huge difference, but when you consider that these numbers cover more than 1,000 possessions in both cases (and nearly 2,000 in Boston’s case), the percentages end up being very significant.

Another indicator that it’s the Celtic system, as opposed to individuals, is the pick and roll defense. According to Synergy, the C’s allowed opponents just 0.94 points per possession on P&R Roll Man plays, good for 7th overall and used 5.4% time. P&R Roll Man plays are usually higher in PPP (since they often end in baskets close to the hoop), so the fact that the C’s forced the P&R Handler into taking the shot 11.4% of the time is good.

An important caveat, however: In some situations, you better be careful what you wish for. Sure, you would rather have Joakim Noah trying to put in a shot off a P&R than Derrick Rose, but what about Miami? When Bosh runs a pick and roll with Dwyane Wade (or worse: when LeBron acts as the roll man, where he averaged 1.17 PPP), how do you defend that? Who do you want shooting?

There is, of course, no easy answer, or the Heat wouldn’t be nearly as dangerous as they are. No matter who guards Wade/James/Bosh and no matter perfectly how the C’s rotate, there is always going to be a frighteningly good chance that they will be able to blow by to the basket. But one way the Celtics compensated last year, especially when Bosh set high screens, was by having interchangeable players guarding both Handler and Roll Man, then just switching on everything. Rondo v. Wade isn’t a good defensive matchup for this reason; though he is a good defender, Rondo can’t possibly guard Chris Bosh rolling to the hoop if the C’s need to switch.

This is a big part of the reason why Jeff Green’s abilities, particularly defensively, matter so much to the Celtics as title contenders. If Green can switch and play moderate-to-good defense on Miami’s Big 3 (not shutdown defense, but GOOD defense), he is going to be an invaluable piece to the puzzle, a piece that the Celtics badly lacked last year. His length can bother a shooter, his quickness can cut off a drive and his height can bother the roll man. As is so often the case with Jeff Green, he has the potential, but his play hasn’t proved that he can utilize it.

As for the other newcomers? Courtney Lee was a middle of the pack defender last season, but we saw how a middle of the pack defender could be turned into an excellent defender in five years with Ray Allen. Barbosa was much the same way. His numbers are better than Lee’s, but they are also aided and abetted by his proximity to Roy Hibbert in the latter parts of 2012, after Toronto dealt him to the Pacers. And I have made my case for Brandon Bass starting over Jared Sullinger.

We can safely assume that the Celtics will be a very tough defensive team again this year, barring injuries. Whether their stingy defense is stingy enough to take the next step remains to be seen.

Follow Tom on Twitter: @Tom_NBA.