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5/22/2014 4:04 P.M. ET

Casilla out at least a month with hamstring strain

DENVER -- The Giants announced Thursday that right-hander Santiago Casilla, their top setup reliever, is expected to miss at least four weeks with a strained right hamstring.

Casilla injured himself while running to first base during an inconsequential ninth-inning at-bat in San Francisco's 5-1 victory over Colorado on Wednesday. He underwent an MRI on Thursday that revealed a Grade 2 (medium) sprain.

The Giants placed Casilla on the disabled list and recalled right-hander George Kontos from Triple-A Fresno to fill the bullpen vacancy. Kontos appeared in 14 games for Fresno, compiling an 0-2 record with a 3.80 ERA. In 23 2/3 innings, Kontos accumulated 35 strikeouts while issuing only three walks.

Kontos had to leave himself a 3:45 a.m. PT wakeup call to catch his flight to Denver. But, he said, "You're never quite as tired when you come back to the big leagues."

Kontos, who began his second stint of the season with the Giants, was less than thrilled about the misfortune that prompted his return.

"It's a shame that you lose a guy to something as freakish as that," he said.

Losing Casilla threatens to rob the Giants of the bullpen depth that has hastened their success. He had a 1.37 ERA in 22 appearances.

Manager Bruce Bochy said that he will rely on a combination of relievers -- left-handers Jeremy Affeldt and Javier Lopez, as well as righties Jean Machi and Juan Gutierrez -- to handle Casilla's setup responsibilities in his absence.

Cain hoping to make next start despite injury

DENVER -- The Giants received encouraging news about the strained right hamstring sustained by Matt Cain, who will try to make his next scheduled start.

Cain, the Giants' No. 2 starter, underwent an MRI on Thursday that revealed a Grade 1 (mild) strain. Manager Bruce Bochy said that Cain's condition will be re-evaluated in the next few days, leading to a decision regarding the right-hander's playing status. Cain, who went on the DL earlier this season with a cut on his right index finger, is in line to take his next turn Monday against the Chicago Cubs at AT&T Park.

Cain, who hurt himself while throwing a called third strike past Charlie Blackmon to end the third inning in San Francisco's 5-1 victory Wednesday night over Colorado, appeared to walk smoothly Thursday. But, as Cain said, "Just walking around's a whole lot different from playing a game."

If Cain can't pitch Monday, the Giants then would decide whether to return him to the DL or make another attempt to slide him back into the rotation.

Yusmeiro Petit, who followed Cain's three shutout innings Wednesday by blanking Colorado for three more innings, probably would pitch Monday if the Giants need a replacement starter.

Bochy plans to caution relievers about hitting

DENVER -- Giants manager Bruce Bochy said Thursday that he will "take responsibility" for not being more assertive with Santiago Casilla before the reliever made his fateful sprint to first base Wednesday night.

But, Bochy added, pitchers -- especially relievers who aren't conditioned to hitting -- must exercise common sense when circumstances place a bat in their hands.

"It's like each guy's got to touch the stove to find out," Bochy said. Casilla got burned by straining his right hamstring, sidelining him for an anticipated four weeks.

Bochy wished in retrospect that he had ordered Casilla, who was making his fifth career plate appearance, to do nothing more than stand in the batter's box when he faced Colorado's Boone Logan in Wednesday night's ninth inning.

"He was told before the at-bat not to swing," Bochy said. "[But] these guys are competitors and sometimes they forget."

Bochy added that after Casilla hacked at a 2-1 pitch, "I should have called time out and said, 'That's it. No swinging.'"

Bochy plans on reminding pitchers to be smart at the plate and on the bases. Injuries such as Casilla's have occurred too often with Giants relievers when they stray into the batter's box. Left-hander David Huff strained his left quadriceps earlier this season and left-hander Dan Runzler sustained a knee injury in 2010.

"Relievers live for that," Bochy said. "They want that one at-bat."

Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. This story was not subject to the approval of Major League Baseball or its clubs.