KANSAS CITY --- Perhaps no situation illustrated the maturation of Royals reliever Kelvin Herrera better than the sticky mess that he got himself into -- and out of -- last Sept. 29 in Cleveland.

It had been a long season for the Royals and for Herrera, who, on that Saturday night, was making the next-to-last of his team-high 76 appearances. A big workload for a rookie in a season full of big tests.

It was the 14th inning when the Royals went ahead, 7-6, and Herrera was summoned to close it out. Two singles and a walk later, the bases were loaded and the middle of the Indians' lineup was coming up.

"I was trying to keep the ball down, because I wanted to get a forceout at home but I was never nervous. Seriously," he recalled.

Herrera didn't get the forceout, but Carlos Santana flied out to left and Lonnie Chisenhall lined out to right, both too shallow to score the tying run, and Casey Kotchman tapped out to end the game.

A six-game skid was over for the Royals, and Herrera was about to graduate from the season with honors. That was his third save to go with a 4-3 record, a 2.35 ERA and, as a setup man, 19 holds.

All of which puts the right-hander, who just turned 23 on New Year's Eve, right smack dab in the middle of the Royals' plans for 2013.

"I don't think I've ever seen as much growth, or maturity, in a young pitcher as I've seen in this kid this year," manager Ned Yost said after the great escape in Cleveland. "This kid has gone from the beginning of the year, [when] I was afraid to put him in tough situations. I was afraid to put him in bunt situations, because the game just sped up on him real fast.

"There's none of those issues. I've put him in the toughest of situations -- bases loaded or walk guys to load the bases to get a better matchup with one out. Because I trust he can get out of it. ... He's really done a good job of coming a long way."

Herrera was well-aware of Yost's reluctance to deal him a tough test early in the season. After all, Herrera was on a very fast track -- he'd climbed through Class A, Double-A, Triple-A and reached the Majors in just one season, in 2011. And his total Major League experience was two innings in two games. Good reason to be cautious.

"It's a test that every rookie has to pass -- pitching in low-intensity games," Herrera said. "But soon I got confidence, and [Yost] was putting me in more high-intensity situations. And he got confidence in me because I've got more experience."

There was a special bonus for him, too, after he'd missed most of the 2009 and 2010 seasons because of a stress fracture in his right elbow.

"I reached one of my main goals, to keep myself healthy," he said.

The Royals' youthful bullpen got plenty of work this past year because of a starting rotation that too often failed to go deep into games. It wore some on Herrera.

"Sometimes I felt tired, but I saw that my velocity was still there, so I think that was just mental," Herrera said. "Because I'm always ready."

Late in the season, he was using his fastball even more, illustrating something else that experience taught him.

"Everybody has had about 600 at-bats, and so it's a little bit difficult because their swings are slower and it's hard to hit a fastball down and away," Herrera said. "That's when you've got to attack with your strength."

Conversely, early in the season, Herrera had to learn to use his blistering fastball less and his offspeed pitches more.

"It doesn't matter if you throw 90 or 103 [mph]. If you can't change speeds, they're going to sit on a 103 mile-an-hour fastball and drive it," Yost mused in late April, discussing Herrera. "He's got a good curveball, a good changeup. He needs to learn to incorporate both of those pitches in a little more to work the throttle. Then he's going to be successful. But if he just rears back and throws fastball, fastball, fastball -- straight fastball -- he's going to get hammered."

Soon Herrera learned to mix up his pitches, and hitters learned they could no longer just sit on his fastball.

"All of a sudden, a 100 mile-an-hour fastball looks like 120," said Yost.

When it all came together, Herrera was certain that he could weather almost any storm on the field.

"Even if they score a run against me, I feel confident, because I always trust in myself," he said.

Early in his pro career, Herrera was a starting pitcher, and while the Royals might consider that for him again at some point, that doesn't seem part of his immediate future.

"He could definitely evolve into a starter, but he plays such a valuable role in the 'pen, it's something we'd really have to think about," Yost said in September.

Herrera's past arm troubles are an important consideration against putting the extra load of starting on him.

"You put all those things into perspective and try to decide what's best for the organization," Yost said. "He's older and more mature, and his body's filled out more now, which helps things."

After pitching 84 1/3 innings and setting a Royals rookie record for most appearances in a season, Herrera took it easy this winter at his Tenares home in the Dominican Republic. No winter ball. But he had a plan for his conditioning program.

"I'm going to work hard on my flexibility so I can have a good year," he said.

That's what the Royals are counting on.