LOS ANGELES -- Angel Pagan, an integral part of the Giants' World Series triumph last year, will undergo surgery Tuesday on his left hamstring that could sideline him for most of the rest of the season, if not the entire campaign.

Manager Bruce Bochy said Monday that Pagan's procedure, which will be performed here, could shelve the center fielder and leadoff hitter for up to 12 weeks. Under that timetable, Pagan wouldn't play again until mid-September. The Giants' braintrust would have to evaluate at that juncture whether reinstating Pagan so soon before the end of the regular season would be worth the risk of aggravating his injury.

That's exactly what happened last Thursday at Stockton. Beginning a Minor League injury rehabilitation stint with Class A San Jose to test his hamstring, Pagan reinjured himself as he sprinted to first base and was carted off the field.

Pagan, who will turn 32 on July 2, resisted surgery when it was cited as an option after he initially injured himself. The switch-hitter instead devoted his efforts toward healing himself enough to rejoin the lineup. Pagan, who signed a four-year, $40 million contract last offseason, hasn't played since May 25, when he hit a game-winning, inside-the-park home run against Colorado. Pagan has said that he actually hurt himself earlier in that game.

Doctors will attempt to repair the tendon on the inside edge behind Pagan's left knee.

"After getting a second opinion, the consensus was that surgery's the best option right now," Bochy said.

Meanwhile, the Giants will continue filling Pagan's absence with Gregor Blanco, Andres Torres and rookie Juan Perez. Blanco has been particularly efficient as a center fielder, contributing solid defense and batting .305 with a .367 on-base percentage and a .402 slugging percentage in 24 games (including 20 starts) entering Monday.

Nevertheless, Bochy conceded that the Giants will feel the void left by Pagan, who established personal bests in his first season as a Giant last year with 95 runs, 38 doubles and 15 triples, a San Francisco-era franchise record. The Giants obtained him during the 2011 Winter Meetings from the New York Mets for Torres and right-hander Ramon Ramirez. The Giants were 10-14 before Monday since Pagan went on the disabled list.

"It's a loss, there's no getting around it," Bochy said. "He played a significant role in helping win the World Series. Now what has to happen is guys fill in, do a good job and -- I use this term a lot -- soften the blow of losing him."

Pagan was among several key Giants who have sustained significant injuries this season, including second baseman Marco Scutaro (left pinkie), left-hander Jeremy Affeldt (right side), right-handers Santiago Casilla (right knee) and Ryan Vogelsong (right pinkie) and third baseman Pablo Sandoval (strained left foot). Sandoval rejoined San Francisco's lineup in Monday's series opener against the Dodgers.

Pagan ranked among the top performers in this year's World Baseball Classic, batting .364 with a .447 on-base percentage and a .485 slugging percentage in nine games for Puerto Rico.