It’s become an annual rite of passage to discount the Boston Celtics’ playoff chances. The snow turns to rain, SportsCenter begins running baseball highlights (which sucks for us hoop heads) and NBA analysts write off the Celtics, who have limped into the playoffs the last two seasons like Tonya Harding’s goons took a crowbar to their collective kneecap.

This season, it didn’t take so long to write off the Celtics. After a 4-8 start, it was easy to state that their window had finally closed, for real this time.

Yet here we are, on April 10, and if the Celtics aren’t legitimate title contenders at this point, they are at least forcing the world’s finest basketball minds to scratch their heads and wonder, “Can a team with the NBA’s 25th-ranked offense potentially win four consecutive playoff series?”

The Celtics came back to life early this season, and they are doing it almost solely with defense.

Allow Sports Illustrated’s Zach Lowe to explain:

Bottom line: The Celtics’ season-long defense has been very good, but their defense over the last 15 games would qualify as historically good if they can duplicate it over the long haul. And “historically good” at least gives them a chance to be interesting in the playoffs, because the Celtics’ offense has continued to produce at a bottom-five level even during this hot streak.

Boston’s defense has improved in all aspects over the last 15 games, a stretch that has coincided with Avery Bradley seizing rotation minutes and then a starting spot in place of Ray Allen. Over this stretch, the Celtics have fouled less often, cleaned the defensive glass at a slightly above-average rate (an improvement for them) and held opponents to sub-40 percent shooting from the floor and sub-30 percent shooting from three-point range. The only metric they haven’t improved upon over the last 15 games is the rate at which they force turnovers, and they’ve ranked among the top half-dozen teams in that category for virtually the entire season.

The schedule hasn’t been filled with scoring-challenged teams, either. The Celtics have recently faced six of the league’s eight most efficient offenses, though one of those games came against a Bulls team missing Derrick Rose. In the 159 minutes in which Boston’s four best defenders — Rajon Rondo, Kevin Garnett, Paul Pierce and Bradley — have shared the court, the Celtics have surrendered just 81.7 points per 100 possessions, per NBA.com. No four-man group that has logged at least 250 minutes this season has even cracked the 84.0 barrier.

The sample sizes here suggest we have to cool it a bit. Other elite defenses in recent years have gone on similar 10- or 15-game stretches, though a quick scan of NBA.com’s stats database suggest Boston is on one of the best rolls in recent NBA history — and that going on such a roll late in the season is relatively unusual.

What’s changed? Let’s begin with Boston’s starting lineup. By adding Brandon Bass and Avery Bradley into the first five, the Celtics substituted athletic specimens for Ray Allen and Jermaine O’Neal, two of the slower, ground-stuck players at their respective positions. Factor in switching Kevin Garnett from power forward, where he was generally matched up against players at least his equal speed-wise, to center, where he’s now one of the league’s most agile fives, and the Celtics became an athletic team almost overnight after two years of generally being slower than their opponents.

It’s easy to see that Bradley is a natural defensive pest, and Bass, who never had much of a defensive reputation prior to joining the Celtics, has thrived (or been hidden, one or the other) within the Celtics’ schemes. But the most important factor of Boston’s ability to stop opponents remains the intellectually-driven play of Kevin Garnett. Garnett has played well enough to earn Lowe’s consideration for Defensive Player of the Year Award, and Kevin Arnovitz, who has watched “a mile of film” on the Celtics, concurs that Garnett deserves to be in any discussion for the award.

I turn things over now to Arnovitz, who like Lowe did a nice job profiling Boston’s defense today:

Every NBA big man under the age of 25 should have the video coordinator at his team’s training facility make a feature-length DVD of Garnett’s half-court defense. If you watch him closely, you won’t see a lot of blocked shots or pickpocketing. His defensive game is an exercise in nuance. At 35, Garnett could probably defend a pick-and-roll with a blindfold on and his most notable contributions are simply where he situates himself on the court in relation to the offense. Garnett’s hyperawareness of what the offense is trying to accomplish on a given possessions is remarkable. Watch several dozen defensive possessions with this lineup, and you’ll never witness an error in judgment by Garnett. All the while, he’s calling out instructions to his teammates and guiding Bass to the right spots.

Both Lowe and Arnovitz noted the offensive success which Boston’s current starting five has experienced. The unit is scoring 108.2 points per 100 possessions, a rate which would lead the league for offensive efficiency (while surrendering just 81.0 points per 100 possessions, which I’m not even going to discuss beyond this sentence but is certainly an outrageous margin). It goes against all logic that Boston’s new starting unit is scoring at such a solid pace, considering that Paul Pierce is the unit’s only three-point shooter, none of the Celtics tend to draw foul shots very often and the Celtics are one pace to corral fewer offensive rebounds than any other team in history.  But with Bradley functioning as a primary cutter, Bass and Garnett spacing out the floor and Rondo finding his teammates for open looks, the Celtics are jumping out of the gates with offensive precision, if not firepower.

The new lineup has contributed largely to Boston’s recent hot play. Though it has performed admirably with the ball, defense has been its calling card.

John Hollinger delved further into trends which have helped the Celtics establish a historical defensive run during the past 10 or 15 games. (ESPN Insider)

Add a heavy dose of Bradley to the mix, and the result has been that an already excellent defense has become an absolutely terrifying one. In this nine-game stretch, the Celtics have allowed 79 points or fewer five times, and the only teams to beat them are Chicago and San Antonio.

The lineup data supports the idea that Boston has found itself a defensive lineup for the ages. Check out the carnage on NBA.com’s advanced stats tool: When Bradley and Garnett play together, Boston gives up 88.8 points per 100 possessions, allows 38.8 percent shooting and forces nearly one turnover for every assist. This is scary stuff, and it’s not one of those small-minute flukes, either — they’ve played 658 minutes together.

You think that’s impressive? How’s this: When Rondo and Bradley play together, opponents average 82.2 points per 100 possessions.

That’s nearly 20 points below the league average. It’s in 271 minutes, so it’s not as robust a sample as the data with Garnett, but good heavens. The Celtics barely need to bother with an offense if the D is going to provide this kind of domination.

The Miami Heat told the Boston Globe Monday they would never count the Celtics out under any circumstances, and for the first time in three years, NBA stat heads are trending toward agreement. The Celtics aren’t title favorites, but if they keep stopping opponents at historical rates, the Larry O’Brien Trophy isn’t entirely out of the picture.