SANTA CLARA — Aldon Smith was a no-show in the 49ers locker room on Tuesday — just as he has been on the sack list lately.

Through 14 weeks of the regular season, Smith had 19? sacks, enough to join Houston's J.J. Watt in pursuit of Michael Strahan's single-season NFL record of 22?, set in 2001. Smith's campaign was exemplified by his performance against the Chicago Bears on Nov. 19, when he terrorized fill-in quarterback Jason Campbell and sacked him 5? times — a half-sack shy of Fred Dean's 29-year-old franchise record, and 1? short of Derrick Thomas' NFL mark.

A strange thing has happened in the past month, though. Smith stopped getting to the quarterback. Heading into Sunday's NFC championship game at Atlanta, he has gone four games without a sack, the longest drought of his young career.

Why the sudden drop-off?

Monday, 49ers coach Jim Harbaugh seemed offended by the very question.

"Why hasn't he gotten a sack every single game or two sacks every single game?" Harbaugh asked, an edge to his voice. "He's been effective. He's playing effective. That's good. It's what we want. We want to strive for perfection, keep chasing that. But you guys are pointing out every single spot that we haven't quite gotten there."

Others have theories. Some point to Smith's sore shoulder, which has put him on San Francisco's injury report leading up to the past seven games. Could he simply be worn down by a 16-game NFL season?

Others note the recent absence of defensive lineman Justin Smith, who missed 2? games with a torn triceps. It's accepted as fact that Justin Smith's strength and relentless effort take pressure off everyone around him, including Aldon Smith.

Here in Santa Clara, the consensus is that Smith hasn't slowed down at all. Opponents are simply game-planning around him.

"They're changing," 49ers safety Donte Whitner said. "They're chipping him, they're sliding protection to him, they're using two guys. When he and Justin run their little (stunt) game, everybody in the NFL is ready for it now. So it gets harder. When you get to the level that he's at, No. 1 in the National Football League for so long, they're talking about you in the meeting room. They're looking at you on film. They know where you are."

Defensive end Ray McDonald said that opposing quarterbacks are getting the ball out quicker, too, wanting no part of an Aldon Smith sack. Ricky Jean Francois, normally a backup defensive lineman, saw the adjustments clearly when he filled in for Justin Smith, who plays both defensive tackle and end.

"Teams have to respect him," Jean Francois said of Aldon Smith. "While Justin was out and I was on the same side as him, I'd see the frustration of what they gotta go through when you get the center or the guard, the tackle, a tight end or a running back in the back trying to chip him."

It's not like the 49ers' pass rush has dried up during Smith's sackless streak. Jean Francois and McDonald have two sacks each over the past four games. Nose tackle Isaac Sopoaga and linebackers Ahmad Brooks and Patrick Willis have one each.

To hear the Niners tell it, that isn't a reflection of teammates picking up the slack for a diminished Smith in coordinator's Vic Fangio's 3-4 defensive scheme. It's a direct result of all the attention Smith is getting.

"A lot of people just want to see: 'Did he make a sack? Did he make a sack?' " Jean Francois said. "But he's one of the guys that can get the protection to slide, and we can just run wild with the other two guys, or Vic can run a blitz — and you already know that protection's sliding to him, so the backside is free to get it."

Smith has been a phenomenon since he entered the league as a first-round draft choice, No. 7 overall, from Missouri in 2011. He is improbably quick for 6-foot-4 and 258 pounds, and offensive linemen soon learned that he has the power and technique to vary his attack.

Smith led all NFL rookies and set a 49ers rookie record with 14 sacks (he was a half-sack away from Jevon Kearse's NFL rookie mark), then picked up the pace further in his second season. No player has ever had more than Smith's 33? sacks in his first two seasons.

Along the way, teammates say, he has become a better all-around defensive end, too. So even during the shutout, he has been productive.

"As an outside linebacker, when you're not getting sacks, there's other things you can do," Whitner said. "You can set the edge, turn the football back inside, not allow those tight ends and tackles to hook you and bounce the ball outside for 15, 20 yards on the ground. He's been shedding tackles, getting running backs down. Those are other things that he didn't really have to his game last year."

Indeed, Smith has done some good things without reaching the quarterback. He had his first career interception against Tom Brady and the Patriots in Week 15, tied for second on the team with six tackles against Seattle in Week 16, and forced a fumble on a scramble by Packers quarterback Aaron Rodgers in last Saturday's NFC divisional playoff game — a contest in which he played 100 percent of the 49ers' 58 defensive snaps.

Those contributions were important, but it would be huge for Smith to come up with a sack or two against the Falcons, a team that relies on timing throws from quarterback Matt Ryan to his talented corps of receivers. To this point, the no-sack streak doesn't seem to be bothering him.

"If it does, he ain't showing it," Jean Francois said. "He's a team player. And he's going to keep doing what he's doing. I know it might be aggravating because he's like one of the best D-ends or outside linebackers at this rushing game. ... But at the same time, not to have a sack, but to be able to free other guys up? That should be a stat column itself."

(You can reach Staff Writer Phil Barber at 521-5263 or phil.barber@pressdemocrat.com.)