"It scares me to look," Popovich said. "I just know we're significantly lower than we have been in the past."

In 11 full seasons on the Spurs' bench, Popovich never has coached a team that finished outside the top five in field-goal percentage defense, the statistic he considers to be most telling of a team's defensive competence.

One can imagine Popovich peeking at the leaguewide stat sheet through fingers over his eyes, then recoiling in horror at what jumps off the page: The Spurs allow foes to shoot 45.8 percent, 18th best in the NBA.

"We're used to being top two or three every year," Popovich said. "That's obviously not the case right now."

On the surface, there is much to like about the Spurs as they head into the regular season's home stretch. At 35-16, they are four games ahead of the field in the Southwest Division, and have established themselves as a strong No. 2 to the L.A. Lakers in the Western Conference.

But Popovich - whose team, fresh off the All-Star break, resumes its rodeo road trip tonight against the Knicks - knows the Spurs will be judged solely on how deep they can play into the postseason. For that reason, he will continue to nitpick his team toward perfection.

Good thing for the Spurs there is plenty of room, and plenty of time, for improvement.

Four months into the season, they are still breaking in three players new to their regular rotation - Roger Mason Jr., George Hill and Matt Bonner.

Team defense is like a ballet. Each performer must perform his steps exactly right, in perfect time. If one dancer pirouettes instead of plies, "Swan Lake" just doesn't look the same.

As such, the Spurs' most glaring defensive troubles haven't been glaring at all. They have been subtle. A blown rotation here, a failed close-out there. A missed assignment, a brain-lock on a double-team.

"We have three guys still learning the system," Popovich said, "and it shows."

Mason, who has emerged as the starting shooting guard after arriving via free agency during the summer, says the Spurs' defensive system - with its unique rotation rules - is unlike anything he's ever seen.

"It's very different from what other teams do," Mason said. "Like night and day."

Hill, a rookie who has surfaced as the top backup to Tony Parker at the point, is one of the Spurs' top on-the-ball defenders. Yet he still struggles with the team defensive concepts at times.

Bonner, a sharpshooting center in his third season with the Spurs, is further immersed in the team's defensive scheme than the two newcomers to the system.

"There's probably something wrong with me if I'm not at least starting to get the hang of it by now," Bonner said.

Even Bonner, little-used until this season, still has much to learn.

"As he goes through the league, he's learning how to play different guys," Popovich said. "That's something you only learn by playing."

Tonight at Madison Square Garden, against a Knicks team averaging better than 105 points per game, the Spurs' will find a sturdy test for their defense. They also will find a fitting reminder of how well they can play on that end of the court when they put it all together.

On Nov. 11 at the AT&T Center, the Spurs played what Popovich at the time called "our best defensive game of the season," beating the high-octane Knicks 92-80 with both Manu Ginobili and Tony Parker on the bench with injuries.

Popovich hopes for a repeat performance in New York, then another repeat after that, and another after that.

Until that field-goal percentage ranking stops scaring him.

"It's just a process we're going through," Popovich said. "At one point, we were 27th. Hopefully, we can be in the top 10 by the time playoffs come."