Cardinals closer Trevor Rosenthal is only four years removed from being a junior college shortstop, but he has been dominant during the postseason for St. Louis. (Elsa/Getty Images)

In the spring of 2009, the man who will be in position to close World Series games for the St. Louis Cardinals was a shortstop at a junior college in some place called Arkansas City, Kan., just dabbling in pitching. That same spring, the man who will be in position to close those same games for the Boston Red Sox was beginning his first season in the majors at age 34, an import from Japan to shore up the Baltimore Orioles’ rotation.

Trevor Rosenthal’s fastball hits 100 mph. Koji Uehara’s barely breaks 90. Neither had his current job at the beginning of the season. Their paths, their personalities, their pedigrees could scarcely be more different. Yet as the World Series begins Wednesday night at Fenway Park, each is virtually unhittable, and they almost perfectly encapsulate the unpredictable nature of the closer’s role.

“What you realize is having a Mariano Rivera, that’s an outlier,” Cardinals General Manager John Mozeliak said Tuesday. “To try to build your team around a fixture at the end of the game is extremely difficult. Clearly, history shows that it’s not easily accomplished.”

Both the Cardinals and the Red Sox are pursuing their third championship in the past decade, and whichever team wins will do so with its third different closer. Change-up specialist Keith Foulke was on the mound for Boston when it swept St. Louis in 2004, but he was replaced by the powerful Jonathan Papelbon, who finished a sweep of Colorado in 2007. Adam Wainwright was a rookie in 2006 who entered the postseason with three career saves, yet he saved four that October, including the clincher against Detroit. By 2011, Wainwright anchored the Cardinals’ rotation — and he missed that entire season with an elbow injury, anyway — and Jason Motte recorded the final out of the seventh game against Texas.

Motte was supposed to be the Cardinals’ closer this year, yet he didn’t pitch in a game and eventually underwent ligament replacement surgery in his right elbow. Joel Hanrahan was supposed to be Boston’s closer this year, but he struggled through nine appearances before tearing a muscle in his right arm, done for the season. His initial replacement, Andrew Bailey, lost the job, then was lost for the season with a torn labrum.

The St. Louis Cardinals and the Boston Red Sox face off in a rematch of the 2004 World Series beginning Wednesday night in Boston. The Post Sports Live crew predicts which team will win and in how many games. (Post Sports Live/The Washington Post)

The retirement of Rivera — the New York Yankees’ set-your-clock-by-him closer since 1997 — only put a greater emphasis on the fact that, even for the best franchises, it can be difficult to figure out who the closer will be next week, let alone next year.

“It’s probably the most volatile part of the team to evaluate because performance does fluctuate quite a bit,” said Zack Scott, Boston’s director of baseball operations. “The guys who are consistent performers and are healthy consistently tend to get paid for that, and it’s not necessarily the most efficient part of the market. There aren’t that many of those guys that are that dependable, so you sometimes have to find maybe those undervalued players.”

Uehara and Rosenthal, then, would have two things in common. Both were, at some point, significantly undervalued. And both have served to shorten his team’s postseason games. Uehara, 38, has pitched nine October innings for the Red Sox and has allowed five hits and no walks, striking out 13, while allowing opponents a .161 average and a .452 on-base-plus-slugging percentage. Rosenthal, 23, has made seven appearances and thrown six scoreless innings, striking out nine and walking two, allowing an ungodly .136 average and .436 OPS.

Yet it’s extraordinarily unlikely that either ended up in his current spot. Start with Uehara. As a rookie with the Yomiuri Giants, he won 20 games, but he eased into a relief role as he entered his 30s. When he decided to come to the U.S., the Red Sox evaluated him as a potential swingman, a reliever who could possibly make a spot start. The Orioles, though, paid him as a starter — two years, $10 million — and Boston charted his course from afar, even as he struggled through injuries and ended up in the Baltimore bullpen.

Late in the 2010 season, Scott clearly remembers watching Uehara pitch the ninth inning against the Red Sox. Uehara needed nine pitches, eight of them strikes, to get an easy fly ball and two strikeouts.

“It was the most impressive save I’ve ever seen,” Scott said. “Obviously, being in the AL East, we’ve seen a lot of good closers with Mariano and stuff. But it was so impressive. There’s just no drama with him. He just attacks guys.”

And he does it with a fastball that averages a pedestrian 89.2 mph, adding a devastating split-fingered pitch that, catcher Jarrod Saltalamacchia said, “he can make go down, right, or left. His fastball, he hits the corners. You can’t just wait for his mistake and hit his mistake.” Because there are just so few. Uehara pitched a career-high 741 / 3 innings, struck out 101 and walked nine.

“Easy,” backup catcher David Ross said. “If I had to pick one word, catching him is easy.”

The spitfire Rosenthal might be the opposite. His average fastball sits at 97.2 mph, and he hits 100 basically every time out. When the Cardinals sent a scout to watch Cowley County Community College, he had scarcely pitched at all. But as he said, “Luckily, I was gifted with a good arm.”

Uh, yeah. Some franchises had no idea who he was. The Cardinals took him in the 21st round that year. The following summer, they made him a starter. But by late in 2012, he was in the majors as a set-up man, and in the postseason, he was eye-popping — two hits and two walks with 15 strikeouts in 82 / 3 innings pitched.

So even as Rosenthal, who had three saves all year, inherited the closer role headed to the postseason — Edward Mujica, who saved 37 games, posted an 11.05 ERA in September — the question that captivates St. Louis is: Next year, will Rosenthal, armed with four pitches, start or close?

“He has a chance to be a starter if that’s something he really wanted to do,” Mozeliak said. “From a talent standpoint, he has an extremely high ceiling. But when you see him in a short stint, he’s dynamic.”

Both these guys have been — recently. But the history of their two teams, and their personal histories, show that it all can change, and fast.

“You see the turnover that’s been happening,” said Cardinals reliever John Axford, who tied for the National League lead with 46 saves for the Brewers in 2011 — and was out of the closer’s job by 2013. “It’s a difficult role, obviously. The attention that’s brought to it now changes a lot of the mentality and the thoughts of coaches and the city. . . .

“The closer was kind of this aura of a baseball player where they were always going to be in that role. Now, it’s a little bit different. They’ve seen success from some guys for a couple years, success for others who came out of nowhere, like myself, and they know if one guy’s not going to do it, maybe someone else can.”

The part that’s clear: Both Uehara and Rosenthal can, and have. And in vastly different ways, they have provided that most elusive feeling to two of baseball’s most infatuated fan bases: The idea that a lead in the ninth, no matter how small, is safe.