The first few weeks of the NBA season – or any season for that matter – always provide a handful of cheap thrills for stat heads. With a small enough sample size, bench warmers can put up All-Star numbers, and vice versa. Just look at Memphis forward Shane Battier, a career 45% shooter current nailing them at a 58% clip.

Some of these improbable stats, however, are harbingers of significant problems. The Nets, who have finished in the top seven in Defensive Efficiency (points allowed per 100 possessions) in each of the last four years, entered the weekend as the fourth-worst team in the NBA in the category.

Using conventional stats, it may not seem like the Nets have any serious issues on the defensive end; after all, they’ve allowed only two of their nine opponents to break the century mark. But that’s often a factor of tempo rather than offensive firepower. Under coach Lawrence Frank, the Nets have routinely ranked in the bottom half of the league in Pace Factor, which measures numbers of possessions per game. This season is no exception; New Jersey is playing at the seventh-slowest pace in the league, allowing an average of just 91.5 possessions a game.

Unfortunately for the fans in the Meadowlands, the Nets are also allowing 105.8 points per 100 possessions, their worst ratio since the dark days of the 2000-01 season, when the team went 26-56 with a Defensive Efficiency rating of 106.6.

So what’s going on? Is Jason Kidd growing old before our eyes? Did Richard Jefferson’s injury, which cost him most of last season, turn him into a mediocre defender? Is the hype about Nenad Krstic being a European banger all hot air? No, nope, and uh, no. The Nets’ starters are still playing defense at a high level. If you’re going to point a finger, point it squarely at the bench.

For years, the Nets’ bench was an offensive sinkhole (do the names Anthony Johnson, Brian Scalabrine, and Jaime Feick ring and bells?), and it was the reason why a team with three first-rate offensive players – Jason Kidd, Richard Jefferson, and Vince Carter (and Kenyon Martin in the seasons before Carter arrived) – could consistently rank in the lower echelon offensively. On most NBA teams, the reserves account for 25% to 35% of minutes played, meaning a significant drop-off from the benchwarmers will dramatically affect team production totals.

Recognizing the trend, Nets team president Rod Thorn moved aggressively during the off-season to acquire role players who score at a professional level. In adding Scott Padgett, Lamond Murray, Mark Jackson, and Jeff McInnis to holdovers Zoran Planinic and Clifford Robinson, Thorn set out to create a solid second unit to fortify the team’s offense. It’s worked; the Nets currently rank sixth in Offensive Efficiency at 105.9. Unfortunately, most of these players have worse defensive reputations than any random trio of Knicks, and according to the plus/minus data at www.82games.com, most of the Net reserves are playing down to those poor reputations.

Per 48 minutes, the Nets are 26.1 points worse defensively with McInnis on the floor, 13.5 worse with Robinson out there, 10.1 with Planinic, and 8.1 worse with Jackson. Those four players are the team’s principal reserves; they combine to average 64 minutes of court time per game.

Some of the trouble was obvious during a 110-96 opening-night loss to the Milwaukee Bucks. The Nets starters ran up a 16-point first quarter lead, only to watch the reserves fritter it away. When the reserves entered to give Kidd and Co. a breather in the second half, the Bucks went on a 26-7 run to put the game away. During the run, Murray and McInnis repeatedly got lost on screen and rolls, and Planinic was slow in help defense situations.

In general, Frank likes to run the reserves as a unit with Jefferson. But with the backups providing little if any defensive resistance, he has had to concoct new personnel strategies. For one, Frank has gradually shifted away from playing the reserves as a unit and toward a mixing and matching of front and backcourts, playing Jackson, Robinson, and Planinic with Kidd and Carter, for instance.

Saturday night’s 89-83 win over Washington showed Frank’s new alignments paying dividends. In the second period, a lineup of Jefferson, Carter, Kidd, and McInnis repeatedly harassed and pressured the Wizards’ ball handlers, forcing turnover after turnover as the Nets took a seven-point lead they wouldn’t relinquish By halftime, the Nets’ defense had coerced the Wizards into 12 turnovers, an impressive feat for a team that is only forcing 14.2 per game, and extraordinary since the Wizards are only giving away 13.2 per contest.

Unlike perennial defensive stalwarts Detroit and San Antonio, the Nets defense doesn’t rely on an intimidating presence in the middle; New Jersey shuts teams down by disrupting the flow of the offense on the perimeter, which then forces opponents into poor shot clock management and turnovers. Kidd, Jefferson, and Carter are quick to double-team inside and get back to their perimeter assignments, but the reserves have been less sticky to their own men and much less able to help on other attackers.

The downside to Frank’s new alignments is that it forces the starters to play more minutes, particularly until starting pivotman Jason Collins returns to the lineup. Right now, Robinson is the only bench warmer making a consistent effort to deny his man the ball in the post and crash the boards aggressively. Perimeter defenders like Planinic, Murray, and McInnis are frequently beaten off the dribble, and they’re slow at help assignments inside, a cornerstone of the Nets defensive schemes.

Last year’s reserves were awful offensively, but at least they executed Frank’s defensive game plan. The newcomers may need time to adjust to Frank’s system (last year’s Nets weren’t stellar defenders for their first few weeks, either), but if they don’t, all of Thorn’s moves will amount to elaborate water treading.

The Nets’ schedule for the rest of the month features a five-game road trip to the West coast, followed by a home game against Detroit. If their defense doesn’t return to something resembling its old form, the Nets could easily enter December looking more like a team struggling to win a weak division rather than one trying to reclaim its place among the league’s elite.

mjohnson@nysun.com