But when he reached the mound, Reds manager Dusty Baker hands Herrera -- gathered infielders towering over him -- the ball in a 5-5 game against the White Sox.
The game is now in Herrera's hands.
His Reds cap pulled just above his eyes, Herrera coolly nods to catcher Ryan Hanigan.
Despite his 5-foot-6 frame, Herrera is anything but intimidated.
That brazen confidence won't come in the form a blistering fastball that many middle relievers use in tight situations. Herrera certainly won't blow away White Sox hitter Chris Getz.
His first pitch comes in at a mere 84 miles per hour. Ball one. And strike one comes in even slower at 83 mph. His third pitch reaches the plate at an unhurried speed of 75 mph.
But Herrera doesn't need a blazing fastball to get Getz out. Two pitches later, Getz weakly grounds out to second.
Herrera now walks toward the dugout, and all the fans who questioned who this diminutive figure was, realize, that's no bat boy at all.
Yet this type of performance wasn't unexpected. Herrera has made appearances like this one many times this season. With a 2.05 ERA, he has been called upon whenever his team needed a boost.
"I'm pretty cocky on the mound you could say," Herrera said. "Every pitcher has to have it. I don't have all the physical abilities, but the mental abilities have carried me a little bit."
Baker has certainly noticed those mental abilities. As the season has rolled on, he has relied upon Herrera more and more in tough situations.
"He's fearless, and he's very confident," Baker said. "That's the only reason why he's gotten to where he is now."
Herrera has gotten to where he is quickly. In 2006, the Texas Rangers drafted him in the 45th round. Three years later, he's become an integral part of Cincinnati's bullpen.
"I always had hopes and dreams," Herrera said. "But to realize that I'm up here now and looking back on it, I never thought I would be up here already and producing like this. It's been fun."
Herrera entered Spring Training with an outside shot to make the Reds. He knew the club would keep two left-handed pitchers. Veteran Arthur Rhodes was a lock and Bill Bray seemed like the other likely candidate.
But when Bray struggled with an injury to his pitching arm, Herrera became that second left-handed reliever.
"It was a nice surprise," Herrera said. "It's worked out well."
Indeed it has. Herrera has surprised the Reds with how well he has pitched. He has moved from one of the last relievers Baker used in his bullpen to one of the most reliable.
Herrera allowed two earned runs over his first two outings but didn't allow a run over his next seven appearances. On the season, he has allowed seven earned runs in 31 games.
So how does he do it?
With undersized pitchers such a rarity in today's game, Herrera has certainly beaten the odds. Even so-called undersized pitchers like Roy Oswalt (6'0") and Tim Lincecum (5'11") tower over him.
"They probably underestimate me a little bit," Herrera said of opponents. "If they underestimate me at all, it's my advantage."
Another advantage Herrera has is a pitch that few of his contemporaries have mastered: the screwball. He throws this complicated pitch at 67-68 mph and can make hitters look silly. He used the changeup to help him hone the screwball.
"It was trial and error with different grips and different releases," he said. "It took probably about a year and a half to get it through and get it going."
When he arrives on the mound to get the Reds out of a jam or to hold a late-inning lead, Herrera shows nothing but poise. Any hitter who thinks the crafty lefty will be an easy hit is mistaken. And Herrera makes sure hitters know that as he stares them down.
"They say a lot about size, but I've seen guys who look like King Kong and can't get anybody out," Baker said. "Then there are guys like him who look like a cheetah who get everybody out.
"Heart is some of it; stuff is some it -- and command and control is most of it."
Herrera built that command and toughness in Odessa, Texas. Growing up in the city where the book "Friday Night Lights" was based, he was attracted to football. But because he was so undersized, baseball became his focus.
He didn't let his lack of size stop him. He earned a baseball scholarship to the University of New Mexico where he went 10-0 with a 2.24 ERA his junior year.
Those numbers drew the attention of scouts. And in the 2006 First-Year Player Draft his dream came true when the Rangers, his favorite childhood team, selected him.
But the dream didn't last long, for after the 2007 season the Rangers traded him and starter Edison Volquez to the Reds for outfielder Josh Hamilton.
"It was a little heartbreaking," Herrera said.
Yet he didn't let it faze him. In June 2008, he made his Major League debut. Although his initial trip to the big leagues was short-lived, Herrera has made this season's stay last much longer.
"He's a guy who they told me forgets yesterday if it was a bad day," Baker said. "He means a lot."
He means a lot to his teammates too. They may jokingly call him "horse jockey" or "bat boy" but they know his value to the team.
"It's impressive when you see a guy of his height competing out there," Reds reliever David Weathers said. "He's just been a big part of our bullpen."
Herrera doesn't back down from the height jokes.
"You just shrug it off and fire back at them," Herrera said.
And if Herrera keeps firing back at opposing batters the way he does his teammates, hitters won't continue to shrug him off as a "bat boy" when he enters the game.
Steve Gartner is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.