SANTA CLARA (AP) — Aldon Smith hardly has a free moment to stop at his locker, much less give thought to any intriguing matchups or scenarios as he chases Michael Strahan's single-season NFL sacks record.

Yet, come Sunday at Candlestick Park, it won't be the injured Jake Long lining up at left tackle for the Miami Dolphins against the 49ers defensive menace but rather rookie replacement Jonathan Martin. He is a former Stanford player plenty familiar to 49ers coach Jim Harbaugh, defensive coordinator Vic Fangio and the rest of the staff — and the young offensive lineman suddenly faces the daunting challenge of slowing down the NFL's sacks leader.

"I've just got to take advantage of the opportunity," Smith said.

Smith has 17 1/2 sacks, already tying Fred Dean's franchise-best total set in 1983. Next up: getting the 5 1/2 sacks he needs over the final four games to break Strahan's mark of 22 1/2 sacks set in 2001 with the New York Giants.

"What does he have, 17½ sacks?" Dolphins coach Joe Philbin said. "In 12 games, that's quite a pace he's on. He's kind of got a knack for getting to the quarterback and sniffing things out. He's a good rusher when the quarterbacks drop back, he does a good job containing when the quarterbacks try to bootleg his way, he does a good job hustling and chasing down quarterbacks when they're maybe extending a play.

The 6-foot-4, 258-pound Smith has long arms, a quick burst of speed off the snap and a relentless approach to pass rushing that Philbin described as "some kind of natural wiggle in him as a pass rusher."

"He's chasing history, and he's going to keep doing what he does to make the plays and help the team win," said defensive mate Justin Smith, who has at least a half sack in each of his last three games. "If along the way he gets the record, so be it. We're all pulling for him, hope he gets it, and we're pretty confident he will."

Last week, Smith's sack of Sam Bradford late in the first half at St. Louis gave him an NFL-best 31 1/2 in his first two seasons, a half-sack better than Reggie White's previous record in 1985-86. While Smith has publicly said he wants to be Defensive Player of the Year and top in sacks, he is more concerned with doing his part to put San Francisco in the best position heading to the playoffs next month.

"We'll talk about it when it matters," Smith said.

Smith recorded 5 1/2 of his sacks for San Francisco (8-3-1) against Bears backup Jason Campbell on Monday Night Football on Nov. 19, earning him NFC Defensive Player of the Week.

Every week, Smith shows the rest of the NFL just what a menacing presence he is by making plays every which way. The 49ers selected Smith seventh overall in the first round of the 2011 draft out of Missouri — and he became an instant impact player like many of the others chosen by general manager Trent Baalke the past two drafts.

Dolphins quarterback Ryan Tannehill hopes his encounters with Smith will be few. They faced off during college when Tannehill played at Texas A&M.

"I didn't realize, I guess, the player he is and he turned out to be at the time," Tannehill said. "I knew they had a good defensive end. In college, you study more defenses in general instead of focusing on players and names. You don't put in so much time into that, but I definitely know who he is and we'll be looking out for him."

Smith's matchup against Martin is far more favorable than if he were facing Long, who was placed on season-ending injured reserve Tuesday with an injured left triceps.

Not that Smith thinks making sacks will be any easier.

"I think if you get wrapped up in something that's on the periphery that sometimes you don't have control over, sometimes it could be a distraction," Fangio said. "I think he's handled it pretty good so far. Every week in the NFL is a tough assignment no matter who you're going against."

Philbin has no concerns about Martin being ready for the task Sunday — and the 49ers coaches who know Martin well from his Stanford days realize he wouldn't be put in such a position if he couldn't handle the load. He protected No. 1 pick Andrew Luck's blindside for three years in college, after all, before Miami selected him 42nd overall in the second round.

"This guy hasn't batted an eye," Philbin said. "I mean he walked in here and has been a starter since the day he got here basically, and he just keeps working. He doesn't say a whole lot, he just works and he's diligent."

Notes: WR Mario Manningham, nursing a shoulder injury, isn't sure whether he will play Sunday. "Just taking it day by day, just see how it feels every day," he said. ... CB Chris Culliver said he was limited Wednesday because, "I had the sniffles, still getting over it."... The 49ers celebrated the placing of the highest gold-painted steel beams — about 200 feet above the ground — in the construction of their new stadium adjacent to team headquarters. The $1 billion stadium is set to open in 2014.