Zito was removed from Saturday night's 5-3 victory over the D-backs in the second inning. X-rays were negative, and he underwent an MRI on Sunday that substantiated the extent of the injury.
"We're going to have some more tests done," Giants manager Bruce Bochy said on Sunday before the Giants concluded their three-game series against the D-backs at Chase Field. "We'll have a little more information about how long this is going to be. But there's no way he's going to be ready to pitch in two weeks."
Zito left the clubhouse at Chase Field on crutches with a swollen ankle after the game, which extended San Francisco's current winning streak to four in a row. His personal streak of 356 regular-season starts without having missed one because of injury is now at an end. Zito was next slated to start against the Braves in San Francisco on Friday night, and Bochy said Vogelsong is tentatively slated to fill that slot.
The Giants also had been on a roll without a member of the five-man staff missing a recent start because of injury. The Giants used only seven starters last year when they won the World Series for the first time since 1954, and had been using the same rotation since Madison Bumgarner was brought up last June 26.
"Knock on wood," Bochy said on Saturday. "They key to our success is that they've made their starts. It's tough when you have to make adjustments to your rotation. [Until now] we haven't had to do that."
With the addition of Vogelsong, the drop off may not be very extreme. He was 2-0 with a 1.59 ERA and 17 strikeouts in two starts this season for the Grizzlies in the usually hitter-friendly Pacific Coast League.
As far as Zito's injury is concerned, while Arizona rallied in the bottom of the second inning on Saturday from a 2-0 deficit, Zito dove forward to catch a pop bunt by D-backs pitcher Joe Saunders in front of the mound. Zito made the catch but as he readied himself for the next hitter, Giants athletic trainer Dave Groeschner came to the mound at the urging of catcher Buster Posey.
After some discussion, Zito took a warmup pitch and then walked off the field accompanied by Groeschner and manager Bruce Bochy. Zito had already allowed a pair of doubles in the inning, the second of which, struck by D-backs catcher Miguel Montero, drove in two runs.
Zito said he felt his foot land strangely on the turf as he tried to push off to make the play, although he didn't twist it.
"I was just pushing off to get the bunt," Zito said. "My foot went at a weird angle. I threw the pitch, saw the ball come off the bat and went to go get it. It's really odd. I put my weight all on one foot just to jump."
Zito is 0-2 with a 6.23 ERA in three starts this season. He hasn't won since Sept. 19 in San Francisco over the Brewers and was left off last year's postseason roster for all three rounds.
Zito is earning $18.5 million this year in the fifth year of a seven-year contract that guarantees him $126 million. Since joining the Giants for the 2007 season, Zito is 40-58 with a 4.48 ERA.
This hasn't been a good season thus far for Zito. He was in a car accident in Los Angeles on the eve of the March 31 season opener at Dodger Stadium. Though he sustained a neck injury in the collision that totaled his rental car, Zito made his first start as scheduled on April 3 during that four-game series. He threw six innings that day in a 7-5 loss.
"I was able recover to from that accident, and it was huge," Zito said.
Barry M. Bloom is national reporter for MLB.com and writes an MLBlog, Boomskie on Baseball. Follow @boomskie on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.