The Padres called upon Guerrero, their No. 13 prospect, to make his debut in the eighth inning of a 5-1 defeat to the Giants on Tuesday night at Petco Park. Shortly thereafter, Posey would become his first conquest, as Guerrero induced a fly out to right with his first pitch.
"I was really nervous warming up, and when I was facing Buster Posey, I was definitely nervous, too," Guerrero said through a translator. "But after I threw the first pitch, I calmed down."
That first pitch was a 96-mph fastball -- and there's more where that came from with the lanky 6-foot-8 Guerrero.
"I'm definitely a fastball-heavy pitcher," he said. "Tonight, I was just trying to give it my best. [Ninety-six mph] is just what I hit. I think you can expect more."
On the night, Guerrero allowed one run on three hits in two innings, keeping the rest of the Padres' bullpen fresh.
A native of Boca Chica, Colombia, Guerrero has been pitching in the Padres' organization since the 2010 Dominican Summer League. In 11 appearances for Triple-A El Paso this season, he posted a 3.60 ERA with 11 strikeouts in 10 innings.
Guerrero got the callup Sunday with the club in need of bullpen reinforcements in Milwaukee, but hedidn't pitch.
"A Major League debut is fun for anybody," said Padres manager Andy Green. "He got two innings in -- we needed somebody to take down two innings right there. That was big for us. I think you saw where he can be effective. That fastball really plays."
There's life on the fastball; no one denies that. But the issue with Guerrero has long been his control. He's averaged 6.1 walks per nine innings in the Minors.
Guerrero's first big league walk was a four-pitch free pass to opposing pitcher Madison Bumgarner. Ultimately, Green came away impressed with the Guerrero's outing -- with that one caveat.
"Attack Bumgarner a little bit more aggressively," Green said. "You've got 96, you've got life, trust it. You don't need to pick around the corners."