The Giants are in a bit of an identity crisis this season. A team that rode starting pitching and defense to World Series championships in 2010 and '12 has suddenly lost both.
San Francisco entered Friday last in the National League in starting pitching ERA (4.75) and fielding percentage (.981), as it has fallen toward the bottom of the NL West.
The Giants did get a quality start from Matt Cain on Friday but they made two more errors to drop the opener of a three-game set with the Cubs, 3-2, at AT&T Park.
"What helps you win games is pitching and defense," Giants manager Bruce Bochy said. "We're all disappointed with where we're at. It's not something that can't be overcome. It's not so much where we're at as much as getting back to who we are as a club."
Left-hander Madison Bumgarner will try to help the Giants find their groove Saturday against Chicago lefty Chris Rusin. Bumgarner will have a few trends working against him. The Giants have lost five of their last six and are just 7-17 against the NL Central and 17-35 against non-NL West teams this season.
Cubs: Rusin steps in for Garza
Rusin will make his third start of the season for the Cubs, filling in for Matt Garza, who was traded to the Rangers this week.
Rusin made his Monday start on short notice, flying in that morning from Dallas after getting word that Garza had been dealt. He got the win over the D-backs after tossing five innings, allowing two hits and two earned runs.
In 19 appearances (18 starts) with Triple-A Iowa this year, Rusin went 8-7 with a 3.35 ERA. The 26-year-old has never faced the Giants in his two seasons in the Majors.
• The Cubs selected the contract of right-handed pitcher Eduardo Sanchez from Iowa on Friday to add a fresh arm to the bullpen and fill the roster spot left by the Alfonso Soriano trade to the Yankees.
Sanchez, 24, was available for Friday's series opener against the Giants but did not pitch.
He was claimed off waivers from the Cardinals on May 21 and has gone a combined 1-1 with one save and a 3.71 ERA in 28 relief appearances between Triple-A Iowa and Memphis.
Giants: Bumgarner rolling
Despite San Francisco's starting pitching troubles this season, Bumgarner has been steady and effective.
Bumgarner brings a 10-6 record and 2.93 ERA into Saturday's start against the Cubs. He has won three of his last four starts, with a loss coming on a seven-inning, one-run start against the D-backs on Sunday.
The left-hander is 4-2 with a 2.78 ERA in seven career starts against Chicago.
• The Giants acquired Minor League right-handed pitcher Guillermo Moscoso from the Cubs on Friday in exchange for a player to be named or cash considerations.
The 29-year-old has appeared in the Majors with Texas, Oakland and, most recently, Colorado, pitching to a 11-12 record and 4.16 ERA in parts of four season. He allowed 186 hits in 192 2/3 innings during that span.
In 17 starts with Triple-A Iowa this season, Moscoso went 7-5 with a 3.93 ERA and struck out 94 batters in 94 innings.
• San Francisco activated Joaquin Arias from the 15-day disabled list Friday and in a corresponding move, optioned reliever Jake Dunning to Triple-A Fresno.
Arias had been on the DL since his July 8 appendectomy and entered Saturday hitting .282 with a .302 on-base percentage and 10 RBIs in 60 games this season. He has appeared at shortstop, third base and first base.
• The Giants have won three of five against the Cubs this season and have won nine of their last 12 against the Chicago overall.
Kevin Massoth is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.