GLENDALE, Ariz. -- It was difficult hearing Todd Bowles amid the celebratory clamor in the Cardinals locker room Sunday afternoon. The Arizona defensive coordinator spoke so softly his words arrived with the force of cotton balls hitting the floor. The same could not be said of the statement his players made 20 minutes earlier, shutting out the 49ers in the second half to facilitate a 23-14 victory that kept the Cardinals undefeated and alone atop the NFC West standings.

The defense's performance through the first three weeks might be the most improbable, yet impressive, on-field story of the young season. Thought to be decimated by the free-agent departure of inside linebacker Karlos Dansby, the year-long suspension of inside linebacker Daryl Washington, the season-ending loss of defensive end Darnell Dockett to a knee injury, and the possible retirement of edge rusher John Abraham (concussion issues), the unit has yet to surrender more than 17 points in a game and has not been scored upon in the fourth quarter.

There's no way of knowing whether Arizona's defense can play at this level for the entire season. After all, you don't lose your best cover linebacker, best rush linebacker, best interior pass-rusher and best edge rusher and not feel it at some point; and the threat of another injury is always a snap away, as witnessed Sunday when starting cornerback Antonio Cromartie did not return after leaving in the first half with a knee injury. But for now, players have adapted and adjusted.

"We know that bad things can happen, but you can't blink," said second-year cornerback Tyrann Mathieu, who returned Sunday for the first time since tearing knee ligaments in December. "We just believe -- believe in each other and our coaches, and what they're teaching us."

The professor is Bowles, an aggressive playcaller who neither makes nor accepts excuses. His ability to accentuate players' strengths while minimizing their weaknesses has been central to a defense that ranks second in points allowed per game, fourth against the run and seventh in total yards.

For instance, he knew that Dockett, an every-down talent who possesses the size and strength to play the run and the athleticism to rush the passer, couldn't be replaced by one player. So Bowles has used the stout 6-foot-6, 310-pound Tommy Kelly to play the run in the base defense, and the athletic 6-3, 280-pound Frostee Rucker in passing situations.

"We lost front-line guys, but [head coach Bruce Arians] and general manager Steve Keim have done a great job building depth," Bowles said.

Some of Bowles' philosophies are so sound and simple it's a wonder more coaches don't use them. Unlike some coaches who use a one-size-fits-all approach to teaching, Bowles is quick to point out that everyone learns differently. Hence, he wants position coaches who are strong communicators who know how to reach players on different levels.

"Some guys get it by film, some guys you've got to put it in ABC's and some guys have to experience it," he said. "We have a good contrast of trying to relate to that guy and see how he learns and then helping him learn that way, instead of saying, 'Well, he can't do this, so he can't play.' We say, 'What can he do?' And if that's greater than his weakness, then we focus on that [and figure out when and where to use him]."

To a man, Cardinals defenders praised Bowles' strategic skills. The 49ers caught the Cardinals off guard in the first half by going with a hurry-up attack early, spreading the formation and using underneath passes when Arizona was expecting run plays. Quarterback Colin Kaepernick capitalized by rushing for 45 yards in the first two quarters, but gained only 9 in the second half, during which San Francisco managed just seven first downs after recording 13 in the opening half.