Posey, recalled from Triple-A Fresno prior to Saturday's game, went 3-for-4 with three RBIs to lead the charge as San Francisco beat Arizona, 12-1.
"He was hitting well [in Fresno] and carried it into the game tonight," Giants manager Bruce Bochy said. "It was a good debut for him. It was something we needed, too. I think it gives us another bat in that lineup. They all swung the bats well tonight."
Posey, the team's No. 1 Draft Pick in the 2008 First-Year Player Draft, is without question the team's catcher of the future. But with the Giants struggling for offense -- entering Saturday 25th in the Majors in runs scored (192) and 17th in batting average (.257) -- San Francisco opted to promote Posey to add a spark in its lineup, inserting him at first base.
And although the 23-year-old prospect did just that with his three-RBI day, he was only one part of a suddenly potent San Francisco offense.
Even before Posey's arrival, the Giants' lineup was already quietly improving.
After scoring only one run in a three-game series last weekend against Oakland, the Giants -- at least the 2010 roster -- were in the midst of an offensive tear, averaging 5.6 runs per game in their past four games, three of which were wins.
On Saturday, the offense erupted in unprecedented fashion behind Posey.
The Giants opened with two runs in the first inning, including one on a run-scoring single by Posey, his first career RBI. As Posey rounded first, first-base coach Roberto Kelly whispered into the young prospect's ear, causing him to crack a smile.
"He was just joking, 'It's that easy, huh?'" Posey said. "And it's not."
The first inning proved only the beginning for Posey and the Giants.
San Francisco leadoff hitters reached base safely in five out of eight innings, and the 3-4-5 trio of Pablo Sandoval, Aubrey Huff and Juan Uribe combined to go 8-for-11 with a homer, four RBIs and eight runs scored.
Uribe alone went 3-for-3 with a solo home run, two RBIs and a walk, and even backup catcher Eli Whiteside got into the mix, hitting a solo homer in the bottom of the fourth inning.
By the time it was all said and done, San Francisco scored two or more runs in four innings as nine different Giants -- including reliever Denny Bautista -- combined for 16 hits en route to a season-high 12 runs.
As the Giants continued to revel in their recent offensive prowess, the team's true strength -- pitching -- continued to impress.
Starter Jonathan Sanchez (3-4, 2.90 ERA) earned the win, but was pulled following the fifth inning after throwing 103 pitches. The left-hander held Arizona to two hits, one a home run by Rusty Ryal, and struck out seven.
The Giants bullpen continued Sanchez's chokehold on the D-backs, as Sergio Romo, Dan Runzler and Bautista combined to no-hit Arizona the rest of the way.
But as Huff -- who walked around the clubhouse jokingly referring to Posey as "Jesus Christ" afterward -- said, the rest of the Giants were just following Posey's lead.
"You know he can hit," Huff said. "I saw him in Spring Training and he's a good kid, has a quiet demeanor about him, does his work and just takes good approaches up there."
Posey, who said before the game he barely slept Friday night after receiving news of his callup, said the butterflies in his stomach were at a minimum.
"I felt pretty relaxed," Posey said. "I was maybe [nervous] a little bit before the game, but once the game started I felt pretty good."
The fans, who have long been clamoring for the top prospect's arrival, also made him feel at home.
When he was introduced on the AT&T Park scoreboard before the game, Posey received a loud ovation.
Through the night, it never quieted.
"That was great. It was fun," Posey said. "It's humbling and that's the time when you really have to try to slow [yourself] down because that can get you going a little bit."