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5/24/2013 11:19 P.M. ET

Giants mulling surgery for Casilla's knee

SAN FRANCISCO -- Giants manager Bruce Bochy said Friday that surgery is a "strong possibility" for right-hander Santiago Casilla, who's sidelined by an injured right knee.

Bochy added that if Casilla undergoes surgery to ease the discomfort caused by a cyst in his knee, the setup man probably will be sidelined for four weeks.

The Giants also are considering scenarios for Casilla to undergo treatment that wouldn't involve surgery. In these cases, Bochy said, he might be sidelined for only slightly more than two weeks. But surgery might cure his ailment for good.

The 32-year-old Casilla, who signed a three-year, $15 million contract extension during the offseason, was 3-2 with a 1.89 ERA, one save and a .167 opponents' batting average in 21 games before he was moved to the 15-day disabled list Tuesday.

Giants nearing decision on replacement starter

SAN FRANCISCO -- Manager Bruce Bochy said that the Giants likely will name a fifth starter for Tuesday's game at Oakland by Sunday.

Bochy reiterated Friday that he and the club's braintrust were still evaluating their options regarding who would replace right-hander Ryan Vogelsong in San Francisco's rotation. By Sunday, whoever that pitcher is will have to intensify his preparations to face the A's. Or, if Chad Gaudin is the choice, the Giants might have to alter how they use their bullpen to account for the long reliever's switch.

Besides Gaudin, possibilities include summoning left-hander Mike Kickham, right-hander Chris Heston or right-hander Shane Loux from Triple-A Fresno.

During an interview Thursday on KNBR, the Giants' flagship radio station, general manager Brian Sabean also mentioned the possibility of starting Gaudin only on Tuesday to buy time for naming a longer-term replacement. Sabean added that the Giants could be on the lookout for serviceable Triple-A starters with contracts that enable them to become free agents by an upcoming date if they haven't been promoted to the Majors by then.

Tuesday will be the first day that the Giants need a fifth starter since Vogelsong fractured the pinkie on his throwing hand last Monday.

Zito plans to change little vs. Rockies

SAN FRANCISCO -- Barry Zito avoids overthinking when he faces an opponent in back-to-back starts, as he'll do Saturday when he faces the Colorado Rockies for the second time in a week.

"You can look back to certain things that you want to do differently, but for the most part, you can't change too much," Zito said. "These guys know what I've got and I know what they've got. Know what I mean?"

Zito definitely means to improve upon last Sunday's appearance at Colorado, where he absorbed the decision in the Giants' 5-0 loss. Zito allowed all of the Rockies' runs as well as 11 hits in 5 2/3 innings.

Zito has faced the same opponent in consecutive starts four times since he joined the Giants in 2007. He has had mixed success in his encore outings, posting a 1-2 record with a 3.97 ERA. Included in those four appearances were two quality starts and two games in which he lasted 3 2/3 innings.

Overall, Zito has thrived against the Rockies, whom he has faced 22 times in his career. The left-hander has built an 8-3 mark with a 2.70 ERA in those confrontations.

Chris Haft is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.