San Francisco Giants pitcher Barry Zito looks down after giving up a solo home run to the New York Mets Angel Pagan in the first inning of their MLB National League baseball game in New York
San Francisco Giants pitcher Barry Zito looks down after giving up a solo home run to the New York Mets Angel Pagan in the first inning of their MLB National League baseball game in New York August 14, 2009. REUTERS

The defending World Series champion San Francisco Giants suffered a double injury blow on Monday with the loss of two key players set to hamper the team's chances of a title repeat.

All-Star second baseman Freddy Sanchez, who dislocated his shoulder on June 10, will undergo season-ending surgery on Tuesday, while starting pitcher Barry Zito is returning to the disabled list because of a nagging right foot sprain.

Sanchez, a .289 hitter this season and an infield mainstay, had been working his way back to full fitness in the hope of returning to the line-up but was unable to restore comfort in his throwing shoulder.

"We were still optimistic that he could find a way to rehab and get to a point where he could help us," Giants manager Bruce Bochy told the team website (sanfrancisco.giants.mlb.com).

"But as each day went by, we knew his chances were diminishing. It's at a point where we have to make a decision to make sure he's ready for next year."

Zito was sidelined earlier this season with his foot injury, but came off the DL on June 28 and strung together a few solid starts before losing form.

Compiling a modest 3-4 record and a 5.62 ERA this season, the left-hander has allowed 19 runs in 15 2/3 innings over his last three starts, walking eight batters.

National League West division-leading San Francisco (61-48) will activate left-hander Jonathan Sanchez to fill the void left by Zito, who has not been given a return date.