Center fielder Angel Pagan will undergo surgery in Los Angeles on Tuesday to repair the hamstring tendon behind his left knee and will be lost for up to 12 weeks, manager Bruce Bochy just said. If so, Pagan could be lost for most or all of the regular season. Twelve weeks from tomorow is Sept. 17, when the Giants have 12 games left in the regular season. Bochy said if all went well, Pagan could return a couple of weeks sooner. “It’s a loss. There’s no getting around it,” Bochy said. “When you lose your leadoff hitter, the guy who led off for you all the way through the World Series, and played a significant role helping you win a World Series, there’s no question it’s a loss.” Pagan will have surgery at the Kerlan-Jobe clinic, performed by Dodgers orthopedist Dr. Neal El Attrache. It is not unusual to have another team’s surgeon perform an operation. Bochy said for the Giants plan to have Gregor Blanco be the primarily center fielder, with Juan Perez in the mix. Bochy has had no conversations at all with general manager Brian Sabean about promoting either Roger Kieschnick, a corner outfielder who has hit well all season at Triple-A Fresno, or Gary Brown, the 2010 first-round pick who struggled for the first two months before going on a hot streak. Pagan first injured the hamstring May 25, he said, while chasing a flyball. He ended that game with an inside-the-park home run. Pagan rehabbed for a month and was hoping to rejoin the Giants over the weekend after two minor-league rehab games. But in the first game he went down in a heap running toward first base on a grounder and aggravated the injury. Also, Bochy confirmed that Mike Kickham will start Tuesday’s game in place of the injured Chad Gaudin. Although Gaudin likely will go on the disabled list, that is not fait accompli. He finally played catch this afternoon and felt much better.