Don’t let Sunday night’s win over Portland fool you: The Knicks have a major problem, and if they don’t fix it they’re going to end up in the draft lottery.

New York squeezed out a 113-105 victory against the Blazers, but that was hardly cause for celebration considering it was at home and the opponent had a pair of teenagers filling in for two missing starters. Similarly, tonight’s gimme at the Garden against the woeful Hornets shouldn’t be used as a litmus test for the Knicks.

The previous four games offered a more telling picture of where this season is headed. The Knicks dropped all four to playoff-caliber teams, culminating in Saturday’s 104-79 beat down in Cleveland. Check the box scores from those games and it’s easy to recognize the underlying issue: The Knicks don’t play any defense. Sunday’s win was just as good an example as those four losses in reinforcing this point; New York allowed a battered Portland team to shoot 55% and only won by shooting 59% themselves.

The Knicks’ unimpressive play during that stretch epitomizes their entire season. I measure a team’s defensive performance using a stat called Defensive Efficiency, which determines the number of points a team allows per each 100 possessions. The Knicks rank 27th out of the NBA’s 30 teams in this category, allowing 105.3. That’s nearly three points worse than the league average of 102.5.

Let’s break it down further. Defense consists of four elements: Forcing missed shots, forcing turnovers, avoiding fouls, and getting rebounds. The Knicks are decent at two of them. They rebound 70.5% of opponent misses, which is just a shade below the league average of 70.9%, and they force turnovers on 15.9% of opponent possessions, slightly above the NBA norm of 15.3%.

In a third area, fouls, the Knicks are moderately below average. New York gives up 0.33 free throws per field-goal attempt, slightly above the league average of 0.32.The reason for that is fairly clear – they’re undersized in the middle, as Shaquille O’Neal made abundantly clear in dominating their contest in Miami. Thus, players have to resort to hacking instead of playing straight-up defense, and Nazr Mohammed and Kurt Thomas each end up ranking in the league’s top 20 in fouls.

The bigger scourge, however, is that the Knicks simply don’t force enough missed shots. Opponents are shooting 46.6%, compared with a league average of 44.4%, a pathetic performance that ranks New York 29th in the league. Only Atlanta is worse, and the Hawks gave up on the season soon after it began.

Again, the lack of size in the middle is largely responsible. But any defense that’s this permissive is a team-wide issue, especially since Kurt Thomas and Mohammed have been the starting lineup’s most dependable defenders. New York’s guards have to pick up more of the slack, especially Stephon Marbury, who has the talent and brute strength to be a defensive terror but seems content with mediocrity. Another weak area is the Knicks’ shoddy defense of the 3-point line. Opponents shoot 36.2% from downtown – compared with the league average of 34.8% – ranking them 22nd. Part of the reason may be the need to sag off and help out the big men down low, but a more substantial portion of the blame lies with the guards’ sloth in rotating out to shooters.

Looking at New York’s roster, it shouldn’t be too surprising that the Knicks have struggled at the defensive end. Isiah Thomas has built what is basically an offensive team. Among the five starters, only Kurt Thomas is known primarily as a defender. Mohammed is a fine rebounder, but doesn’t scare anybody with his shot blocking, while Marbury, Jamal Crawford, and Tim Thomas have been mediocre defenders at best throughout their careers. The Knicks’ D takes another hit with Crawford on the shelf, as Allan Houston is still dragging his leg around the court thanks to off-season knee surgery. The primary frontcourt reserves, Mike Sweetney and Vin Baker, also are chiefly offensive contributors. Stare hard enough and you’ll see the occasional bright spot – rookie Trevor Ariza is the team’s best wing defender; Jerome Williams provides an energetic presence at either forward spot; and Penny Hardaway still can bother shots despite the total dilapidation of his offensive game. But most pro teams wield several quality defenders, not just three, and seemingly everybody but the Knicks has at least one perimeter stopper in the starting lineup.

Here’s the biggest problem: It’s going to get worse before it gets better. The Knicks’ schedule, to date, has been relatively easy. Computer guru Jeff Sagarin of USA Today ranks it 26th, which is another way of saying that most of the good opponents are still to come. Starting next Tuesday, the Knicks face a murderous three-week stretch that includes both their meetings with the 30-4 juggernaut in Phoenix; the Cavs and Heat at home; and road tilts against the Kings, Pistons, Nuggets, Heat, Clippers, Celtics, and Jazz (likely with Andrei Kirilenko back in the lineup). They may be the underdog in every one.

Wait, there’s more: Even with the easy slate thus far, New York isn’t as good as its record. The Knicks have allowed 57 points more than they’ve scored this season, yet are 17-17 overall. With such a negative point differential, they could expect to be 15-19.

As the midpoint of the season approaches, the only logical conclusion is that the Knicks’ $100 million payroll is giving 10 cent results. Thus far, it’s been masked by a soft schedule and good fortune in close games, but with Crawford and Sweetney sidelined and a major spike in schedule difficulty forthcoming, it’s about to get really ugly.

The only thing that can save the Knicks’ season is a massive dose of defensive intensity, but as long as the placid Lenny Wilkens is roaming the sidelines, that seems about as likely as Isiah Thomas showing up to a game in overalls. All of which is a nice way of saying that Knicks fans should enjoy their stay at 17-17 … because it ain’t gonna last.