Whoever said death and taxes were life’s only two certainties never followed pro sports. If he did, a third certainty would have made the list: injuries. In the 82-game grind of an NBA season, it’s virtually a given that somebody — probably somebody important — will be out of commission for at least a portion of the season.

You never know when the injury bug is going to hit, you only know that it’s coming. Normally it doesn’t come until much later in the winter, but just three weeks into the season nearly every team has already lost multiple games from a key player.

As Exhibit A, I present the New Jersey Nets. With Richard Jefferson sidelined by a Grade 2 ankle sprain, New Jersey lost home games against the lowly Sonics and lowlier Blazers, dropping the Nets to 5–4 despite having played only one team with a winning record. With the Nets headed on a fourgame West Coast trip, things might get better before they get worse.

Jefferson has hinted he plans to return soon, but the normal recovery time for this injury is at least a month. While Jefferson is something of a freak (witness his immediate return to action in last year’s playoffs when everybody thought he’d miss multiple games), this is something the Nets shouldn’t mess with. Losing a few extra games in November is one thing; having Jefferson reinjure his ankle and miss several more weeks would be quite another.

Jefferson’s absence is a recent novelty for the Nets, as their “Big Four”of Jason Kidd, Vince Carter, Jefferson, and Nenad Krstic combined to miss only three games to injury last season (they also sat out the final two games to rest up for the playoffs).

Nets fans will insist this was because of the quartet’s incredible durability, but in reality it was because of incredible luck. Kidd, Carter, and Jefferson all have missed chunks of previous seasons with injuries, so a reasonable person wouldn’t have expected the trio to make it through this year unscathed. As it happened, they didn’t go two weeks before the first malady hit.

This is an important point, because it speaks directly to my somewhat pessimistic preseason prediction for New Jersey. While fans usually look at the best-case scenario (i.e., “if everybody stays healthy, we can do big things”), team executives and, yes, prognosticators in the press, are obligated to look at the most likely scenarios, and even at worst-case ones.

In the Nets’ case, it was incredibly likely that one of the Big Four would miss at least some action this year, which is what makes Rod Thorn’s lackluster effort at improving the bench this off-season all the more bothersome. (Brief aside: Thorn is a quality GM, especially with the big-picture stuff, so don’t take this as a “Fire Him Now” rant. But he should have been more proactive this summer.)

We’re seeing the results of New Jersey’s off-season inactivity now that Jefferson is out of the lineup. The Nets only scored 68 points while playing at home against a horrible team Saturday. While Jefferson should return in time for a December schedule chock-full of home games, allowing the Nets to make up lost ground, there’s no guarantee this is the last shock to the system.

They’re not alone. Farther south, the Nets’ nemesis is having similar injuryrelated troubles. Fresh off a championship, Miami made the same decision New Jersey did this off-season, opting not to upgrade a shaky bench in the hopes that everybody would stay healthy again. Oops. Shaquille O’Neal is out six weeks with knee surgery, Jason Williams is just getting his wind back after going under the knife in late summer, and vets Gary Payton and James Posey are already hurting. Not surprisingly, the Heat are 4–5, with half the wins coming thanks to their weird hex over the Nets.

On the other hand, some teams have managed to easily skirt the injury issue because they built a second unit capable of withstanding the increased demands. Look at Utah, for instance, who lost stat-sheet-filling forward Andrei Kirilenko and steady guard Gordan Giricek but responded by ripping off three straight wins last week to improve to a league-best 9–1 mark.

While no team can completely shrug off the loss of a player like AK-47, the Jazz were able to plug in Matt Harpring (quietly averaging 13.1 points on 59.3% shooting) and rookie surprise Paul Millsap (whom the Nets and Knicks both passed on twice in the draft) to contain the damage. Meanwhile the loss of Giricek — who shot 65.2% in the Jazz’s 4-0 start — was offset by solid minutes from wily vet Derek Fisher and rookie Ronnie Brewer.

Or look at the league’s other big surprise, Golden State. The Warriors’ best player a year ago was Jason Richardson, who has been decidedly mediocre thus far in 2006–07 as a result of off-season knee surgery. Richardson and forward Mickael Pietrus have each missed a game, starting power forward Troy Murphy has missed three, and backup power forward Ike Diogu has missed six.

Yet if you watch the Warriors, you wouldn’t suspect anything’s wrong. They’re 7–3 because they’ve been able to plug in talented youngsters like Monta Ellis, Andris Biedrins, and Anthony Roberson into the gaps. Heck, they don’t even play Adonal Foyle, and he’d be getting 20 minutes a night in Jersey.

Of course, the Nets and Heat aren’t the only teams that have trouble. The league’s list of casualties already is daunting — in addition the players I already mentioned, brand-name players like Kobe Bryant, Steve Nash, Pau Gasol, Kenyon Martin, Brad Miller, Josh Howard, Steve Francis, Richard Hamilton, David West, Josh Childress, Bonzi Wells, Bobby Simmons, Charlie Villanueva, Al Jefferson, Brandon Roy, Jerry Stackhouse, and Joel Pryzbilla have missed chunks of time.

So if somebody tells you that the Nets’ struggles are because Jefferson is out, you should only agree to a point. Yes, the problem is technically that Jefferson is out, but the real problem is the failure to plan for Jefferson being out. You can’t base your plans for the season on the idea that everybody will glide through 82 games without incident, and the Nets’ incredible fortune in that department last season appears to have given them a false sense of security. If another sprained ankle or two crops up, it will prove to be their undoing.

jhollinger@nysun.com