Red Sox pitcher Justin Masterson has displayed his mettle in Boston’s bullpen during his rookie season.

The Providence Journal / Glenn Osmundson

ST. PETERSBURG, Fla. — As Justin Masterson stood in the Red Sox clubhouse after a Game One victory in the A.L. Championship Series, a smile didn’t leave his face.

The 23-year-old was answering questions about his clutch pitch to Rays’ star Evan Longoria in the bottom of the eighth inning of a 2-0 game. The Rays had two men on base and Longoria, one of their most dangerous hitters, at the plate.

Masterson’s plan was simple: keep the ball down. Usually it’s his hard sinker that creates the plentiful ground balls that have become his trademark. This time Longoria hit a slider right at Jed Lowrie at shortstop. Lowrie quickly turned it into a 6-4-3 double play that both ended the inning and the Rays’ best scoring chance but also elevated Masterson’s spirits into the stratosphere.

“I was able to stay down in the [strike] zone. That’s all,” he said. “It’s just fun to be a part of this and help us win.”

If you listen to the Red Sox hierarchy, asking Masterson to serve as a bridge to superstar closer Jonathan Papelbon is the exact role they envisioned for Masterson in spring training. Projecting the emergence of a promising prospect, especially a pitcher, is almost impossible in the business of baseball.

But when the Red Sox met in Fort Myers, they told Terry Francona that the strong, 6-foot-6 righthander would begin the year at Double-A Portland as a starter but may be able to help his bullpen this season.

“When they spoke about Masterson [they said that] at some point in the season, he was probably going to accumulate some innings where we may want to put him in the bullpen because he could come up and really impact our season, which is exactly what’s happened,” said Francona. “I think the guys in this organization had a pretty good feel for some of these younger guys.”

A second-round pick in the 2006 draft, Masterson moved to Pawtucket in early July where his starting days ended. He dominated (9 innings, 8 strikeouts, 3 earned runs) in four relief appearances and the experiment worked so well that he was elevated to the majors for good on July 20.

He quickly settled into the role he carries today in the postseason. When the Sox need an out in the seventh or eighth innings, especially with a right-handed power bat at the plate, the rookie will get the call. That’s a lot to ask of any pitcher, let alone a hurler with less than 40 career appearances.

“He’s earned it,” said Francona. “For us to run away from him now would be an error on our part. He did everything we asked of him this year. He started the year great but we sent him to Triple A so he could go to the bullpen so it would be fair to him. He did exactly what he was supposed to do and he’s impacted our bullpen.”

Masterson got the call in the four games of the A.L. Division Series win over the Angels and again in the first ALCS game. He built a 1.93 ERA in 4.2 innings pitched. He’s the youngest Red Sox pitcher to appear in a playoff game since Ken Brett in 1967.

“That’s why you play the game,” Masterson said. “If you’re really into it, you want to be up there when it’s a tough situation, whether you’re a hitter or a pitcher. That’s when it’s fun.”

That confidence and composure is invaluable and what sets relief pitchers apart. Masterson hasn’t been perfect this month, but he’ll take the ball in every game and throw as hard as he can. So far, the results have been exactly what the Sox are hoping for.

“His poise is tremendous,” said Francona.

Masterson says he’s taking in all of the excitement of the playoffs and learning as much as possible from veterans such as Mike Timlin and Javier Lopez. He clearly admires Papelbon (“he’s half-man, half-amazing”) and has the perfect catcher to guide him in Jason Varitek.

“Being around these guys gives you a sense of calmness,” he says. “They make you understand the significance of what we do but you still have to keep your head and execute, too.”

The Red Sox clearly don’t know when Masterson will be pitching next season. He could return to a starting role just as easily as blossoming into the premier set-up man in the game. There are arguments for either direction but everyone agrees this is a good problem to have.

“I don’t think there’s any reason today to clarify what he’s going to do down the road. Our job right now is to win as many games as possible,” said Francona. “I guess when it boils down to it, if you get guys who can get outs, we’ll figure out where they can best impact our team. For the moment, we really like where he’s at.”

kmcnamar@projo.com