Cain isn't able. That means Yusmeiro Petit will get the ball for the Giants on Saturday afternoon, as they look to clinch a series in St. Louis.
Matt Cain was scratched from his scheduled start, as the right-hander remains bothered by a strained right hamstring. Cain was placed on the 15-day disabled list, retroactive to May 22, and manager Bruce Bochy expects Cain to make his next start June 6.
Petit has been versatile for San Francisco, providing the team with long relief when needed out of the bullpen and filling in for injured starters like Cain and Tim Hudson. Saturday's nod will be Petit's fifth start, and while he has experience in multiple roles, his 2014 numbers are better when he's summoned out of the bullpen.
The 29-year-old Venezuelan tossed six scoreless innings in his first start, but in his last three, Petit has surrendered 16 earned runs in 14 1/3 innings. Out of the bullpen, however, Petit's ERA is 2.89 and he's thrown four innings of relief without allowing a run this month.
Petit could potentially see a fresh face in the batter's box, one that Cardinals fans are enthusiastic about.
Oscar Taveras, ranked as MLB.com's No. 2 prospect in baseball, will be active Saturday. Taveras was a late scratch from Triple-A Memphis' lineup Friday, and manager Mike Matheny confirmed he would be added to the roster.
Tavares, 21, is hitting .325 with seven homers, 40 RBIs and an .897 OPS (on-base plus slugging percentage) in the Minors this season.
Michael Wacha will start for St. Louis, looking to add another quality start to what's been two consistent months this season.
Wacha has not allowed more than three runs in any of his 11 outings of 2014 and has gone at least six innings in all but two of them. The righty allowed three runs and four hits in seven innings against the Yankees in his last start.
Cardinals: Grichuk singles after being recalled
Sandwiched between Matt Carpenter and Matt Holliday, 22-year-old rookie Randal Grichuk rejoined the Cardinals lineup Friday after being recalled from Triple-A Memphis.
Grichuk, who had a cup of coffee with the Cardinals earlier in the season, started in right field and singled in his first at-bat in a 9-4 loss to the Giants. The hit was a welcomed sight for Grichuk, who struggled at the plate in his brief 10-game tenure earlier with the big league club, going 3-for-21 (.143) before being sent back down to the Minors.
But once he was back in Memphis, Grichuk was smashing the ball. He hit .321 with seven homers and a 1.029 OPS in a 20-game span between his trips to the Majors, and Grichuk was hoping it would lead to another opportunity.
"Obviously all guys that get sent down want to get up as soon as possible, but I went back down just trying to work on my game, tried to stay focused," he said. "A lot of guys go down there and I feel like they want to get up here so bad they kind of lose focus on the task at hand, and that's playing the game."
Giants: Posey sits another night
Buster Posey, who sat for the second straight game Friday, has started 37 games at catcher for the Giants. Hector Sanchez has started 19 behind the plate, essentially half of Posey's total.
That's probably not the breakdown the Giants envisioned before the season began. They likely envisioned Posey receiving 70 percent to 75 percent of the starts, with Sanchez handling the rest.
Yet the back inflammation that has cut into Posey's playing time hasn't hurt the Giants. They own the Major Leagues' best record, a commanding lead in the National League West and have disposed of the Cardinals twice without Posey's presence.
After being out of the lineup for four games with back-related issues, Posey could return to action for San Francisco's rematch Saturday against St. Louis. Bochy said Friday that Posey felt better and was available to pinch-hit -- or even come off the bench to play, if necessary.
Bochy noted that if Posey had felt too shaky to play defense, the Giants would have to promote a catcher from Triple-A Fresno to serve as a backup.
Meanwhile, Sanchez has benefited from the bonus playing time.
"He's gotten more and more comfortable with the staff, handling and blocking pitches," Bochy said.
Staying unharmed has been an issue for Sanchez, though. He seems to be more of a magnet for wicked foul tips than most catchers.
"We don't get why it's him, but he gets the crud beaten out of him," Bochy said.
• Carpenter extended his hitting streak to 12 games Friday with a 2-for-3 night. His streak is the longest active streak in the National League.
• The Giants built on their best record in the Majors with a fourth straight win Friday. They're now 36-19. For reference, the Giants were 29-25 through May 30 last season.
Stephen Pianovich is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.