Ah, the best laid plans … We’re still nearly two weeks from opening day and already the Knicks and Nets are scrambling to readjust their plans after preseason injuries have forced major alterations to their rotations. And with it, we can see a portent of what’s to come this year.

As luck would have it, each club lost a prized off-season pickup in recent days. For the Knicks it was small forward Jared Jeffries, who will be out sixto-eight weeks with a broken left wrist — ironically, one he suffered in a preseason game against the Nets. The 6-foot-11 forward was the team’s only major addition in the off-season, and he was supposed to be the team’s defensive stopper as the starting small forward.

Instead, he’ll miss the first quarter of the schedule while he recuperates. But there’s a silver lining for the ‘Bockers, as it’s a golden opportunity for Quentin Richardson to prove his back is healed. Richardson struggled with back trouble all last season and suffered through his worst season as a pro, but he’s looked good in camp thus far and there’s little doubt he could competently man the position if he’s at full strength.

In fact, I think there’s a decent chance he might keep the job even when Jeffries comes back. He’s the team’s best outside shooter and would help stretch defenses for Stephon Marbury and Steve Francis, and while he isn’t of Jeffries’s caliber at the defensive end, his willingness to take charges and play physical is in marked contrast to most of his teammates. Right now, the Knicks may need his skills more than those of Jeffries.

Also filling in will be a crew including David Lee, Jalen Rose, and firstround pick Renaldo Balkman. Of the three, Balkman may get the most opportunity just by default. Lee’s minutes will mostly come as Channing Frye’s backup at power forward, while Rose seems to be out of favor due to his age and god-awful defense. Additionally, one has to think Isiah Thomas is anxious to prove he didn’t screw up by taking the unheralded Balkman in the first round, and he may be right — check out that nine-point, seven-rebound, twosteal effort in 13 inspired minutes against Boston on Tuesday.

Regardless of who fills the void, it’s nice to have so many options. The Knicks can shrug off the loss of Jeffries as no big deal because for them, it isn’t. In fact, with the possible exception of Eddy Curry it’s hard to come up with a single Knick whose loss would make you say, “Man, we’re screwed.”

If Marbury goes down, for instance, it’s no biggie — Steve Francis slides over, Jamal Crawford starts, and things are okay. Channing Frye has David Lee, Francis has Crawford and Richardson, and even Curry could be bailed out by an undersized combo of Frye and Lee (but please, not Jerome James).

Contrast that to New Jersey, where any injury to a starter is an immediate crisis, and even the loss of a key reserve is a three-alarm fire. And the Nets lost not one reserve this preseason, but two. The latest casualty was the result of another left wrist injury, as backup point guard Marcus Williams has a sprain that could keep him out past opening day (the team is being parsimonious with details at the moment; in my experience, that’s rarely a good sign).

Williams’s injury exacerbates what is already a problem area for the Nets because another big off-season pickup, guard Eddie House, won’t be back any time soon either. House underwent arthroscopic knee surgery to repair a torn meniscus this week, and the team announced there is no timetable for his return, which loosely translated means, “Check back with us when you’re done Christmas shopping.”

Those two were supposed to be the top backcourt reserves … er, make that the only backcourt reserves. The dangerously thin Nets added the duo in an effort to upgrade what was one of the league’s worst second units last season, but now could play opening day without both.

The only other point guard with the team is Jay Williams, the former second overall pick who is attempting a come back after a horrific motorcycle wreck three years ago. However, his preseason thus far has been a disaster — only two made baskets — and he may be released before the season. Estranged veteran Jeff McInnis also remains on the roster, technically, but the Nets would sooner talk Foots Walker out of retirement than bring McInnis back.

Instead, look for the Nets to pick up a graybeard backup in the next two weeks, perhaps somebody like Anthony Carter (he’s in camp with Denver, but is a possible numbers-game victim on cut-down day). They might do it off waivers or it may cost them a secondround draft pick, but if Williams isn’t back they have to do something. And unfortunately, look for Jason Kidd to play a ton of minutes in the early season as a result.

As for House, remember that he was supposed to be the Nets’ main offensive weapon off the pine. Now New Jersey will have to hope for wholesale improvement from either Antoine Wright or Bostjan Nachbar if they’re going to get any points from the second unit. Good luck with that one. Worse yet, the Nets’ only other important addition is already on the shelf — rookie first-round pick Josh Boone is out with a shoulder injury for the first two months or so.

The injuries aren’t going to go away once Jeffries, House, and Williams return, and in the big picture this is the one area where the Knicks have a huge edge over their crosstown rivals. It’s also a major reason why the two may be much closer in the standings than recent history suggests. New Jersey’s star power and recent success make them seem the overwhelming favorite to reign in the Atlantic Division, but I can’t stress enough how fragile this team is because of the paper-thin bench. This week’s injuries only showcase how much better equipped the Knicks are to withstand an 82-game grind than New Jersey is.

jhollinger@nysun.com