SCOTTSDALE, Ariz. -- Shortstop Omar Vizquel is expected to miss four to six weeks with a torn medial meniscus in his left knee, jeopardizing his availability for Opening Day and causing a ripple effect throughout the rest of the Giants' infield.

Vizquel left camp Tuesday afternoon to return to San Francisco and will undergo arthroscopic surgery Wednesday afternoon to have the torn portion removed. Team orthopedist Dr. Ken Akizuki will perform the surgery, with head athletic trainer Dave Groeschner on hand to supervise.

Groeschner told a club spokesman that the Giants hope Vizquel can return by the first week of the regular season, "but the knee will tell us that."

Manager Bruce Bochy said that Kevin Frandsen will become the Giants' primary shortstop in Vizquel's absence. This interrupts the active competition between Frandsen and Ray Durham for the starting second-base job, although both will continue to be evaluated to determine who remains in the lineup when Vizquel returns. Rich Aurilia, projected to be the top utility infielder, will concentrate on playing third base.

Vizquel's injury jarred a camp that had remained placid without left fielder Barry Bonds' onerous presence. It also challenged the Giants' intention to rely heavily on defense, at least at the season's outset. Vizquel, an 11-time Gold Glove winner, was central to that plan.

"We hope Omar gets back as soon as possible because we know what he brings to this team and this organization," Frandsen said. "All of us have the deepest respect for him, with the way he plays and the way he acts."

Vizquel initially reported knee trouble last Thursday, when he was held out of conditioning drills as a precautionary measure for what was described as inflammation. He declared that the ailment was "no issue," although he was grim-faced as he said so.

Since then, Vizquel abstained from heavy running as he participated in workouts. The swelling around the knee didn't subside and a magnetic resonance imaging examination confirmed the meniscal tear, leading Vizquel and the Giants' medical staff to opt for surgery rather than treatment.

"My knee was feeling good," Vizquel told a club spokesman. "I wanted to give it a [test] in order to make a decision. It didn't work. I felt a little pop when I was running. That's what I wanted to see, if I could sustain the pain."

Vizquel, who turns 41 on April 24, has endured scattered episodes of knee trouble during his 19-year Major League career. He sprained ligaments in his left knee in 1990 and 1992 and needed two right knee surgeries in 2003. He has averaged 150 games per year since then.

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"You've got to think any time a guy has played that long, some wear and tear is going to pop up at some point," Aurilia said.

Vizquel is coming off a season in which he maintained his defensive excellence but batted .246, a 49-point drop from 2006. Needing his glove and believing that he could rebound offensively, the Giants signed him last November to a contract that pays him $5 million this year and carries a $5.2 million option for 2009 with a $300,000 buyout. That year becomes guaranteed if Vizquel plays in 140 games this year -- which no longer seems assured.

Nevertheless, Bochy seemed confident that the Giants would have an able-bodied Vizquel for most of the season.

"When he comes back, he's going to be 100 percent," Bochy said. "When he rehabs it, it should be stronger."

Until then, shortstop belongs to Frandsen, whose Major League experience at the position encompasses two starts in 2006 and 15 last year. Frandsen also played shortstop in the Minors and in the 2006 Arizona Fall League.

"It might be a little different not having 'O' out there, but we'll get it going," Frandsen said.

Yearning for a chance to entrench himself in the Majors after batting .322 in 245 Minor League games, Frandsen's ready to seize this chance -- although he emphasized that it's "not cool" to exult in the wake of Vizquel's injury. But he appreciated the Giants' faith in him: "That excites me. For me, there's no added pressure. It's an opportunity."

Frandsen knows that thriving at shortstop could help him secure the second-base job.

"When Omar comes back, I've still got a spot I need to win and earn," he said.

Frandsen will continue to take some grounders at second and third base -- just as Aurilia will attempt to stay sharp at first, second and shortstop. But third base will be Aurilia's main priority for the immediate future, unless the Giants collaborate with the Chicago White Sox on the much-rumored Joe Crede trade sooner than later.

"If you go into a season knowing you're going to be at one spot, that does take a little bit off your mind," Aurilia said.

Vizquel's injury could increase Eugenio Velez's chances to secure a utility role on the season-opening roster. Already, Velez has been practicing at every position except first base, pitcher and catcher. "He'll keep doing what he's been doing," Bochy said.