The San Francisco Giants have the NL's best record against left-handed starters, though they have never fared well against Jaime Garcia.
The St. Louis Cardinals have captured five straight outings by Garcia heading into Sunday's three-game series finale at AT&T Park.
San Francisco (69-60) is 18-12 when against lefty starters and entered play Saturday with the majors' fourth-best average against southpaws at .271. Nori Aoki is batting a team-best .344 against them.
The Giants could again be challenged by Garcia (6-4, 1.77 ERA), who is 2-1 with a 2.16 ERA in six starts against them. He yielded three runs over 6 1-3 innings Aug. 19 and did not get a decision in a 4-3 victory.
Aoki is 5 for 16 in this matchup and Buster Posey is 4 for 12. San Francisco manager Bruce Bochy may want to start the seldom-used Justin Maxwell, who is 6 for 14 with a homer and three doubles off Garcia.
Garcia is 3-0 with a 1.38 ERA in five starts this month after yielding a run in six innings of Tuesday's 9-1 rout at Arizona. He is 9-3 with a 1.65 ERA in his last 13 road outings.
St. Louis starters are 6-0 with a 1.64 ERA in the last seven games after Lance Lynn pitched into the eighth inning in Saturday's 6-0 win before leaving with an ankle injury that isn't considered serious.
"Just sick when you see him go down like that because he's a tough guy. He has a high pain tolerance," manager Mike Matheny said. "I never thought he was going to be walking off the field. That was huge."
San Francisco's three-game win streak ended one day after it ended a five-game run for St. Louis. The Cardinals broke open a scoreless game with four runs in the fifth.
"The big inning killed us there. We couldn't stop them," Bochy said.
The Giants had a double and six singles.
"We just couldn't do anything offensively," Bochy said. "You look flat when you'renot hitting or scoring."
Stephen Piscotty had four hits Saturday to improve to 6 for 8 in this series. The Bay Area native is 12 for 24 with 10 RBIs during a six-game hitting streak.
Posey was out Saturday after getting hit in the left elbow the day before but expects to be back in the lineup. He is hitting .159 in his last 12 games.
The Giants will start rookie Chris Heston (11-7, 3.34), who makes his first big-league start since allowing one run over 4 2-3 innings in a 104-pitch outing against St. Louis (83-56) in a 2-1 loss Aug. 17.
The right-hander was optioned to the minors after that outing to keep him fresh for the stretch since he has thrown 145 2-3 innings. He did not pitch during his time in the minors.
Cardinals catcher Yadier Molina, who homered off Heston earlier this month, is 7 for 19 with two homers and six RBIs in the season series.
Cardinals closer Trevor Rosenthal remains out on paternity leave.