DETROIT (AP) — When the Detroit Tigers signed Joe Nathan before the 2014 season, they expected baseball's active saves leader to simply keep rolling. After a few too many bad outings, they traded for a backup plan in Joakim Soria, widely considered the top reliever on the market.

That didn't work either — Soria had an oblique injury and by the time the playoffs began, it seemed as though the Tigers had accomplished little with their bullpen.

That's not organizational dysfunction or a problem for only the Tigers. Every team faces the fickle nature of relief pitching — that combination of factors that makes it difficult to predict how late-inning pitchers will perform season to season.

Relievers generally aren't as consistent as starters or hitters, Tigers general manager Dave Dombrowski said.

"You're doing your best to find them. Sometimes guys get injured. Sometimes when they're real good, they pitch a lot," Dombrowski said. "If they pitch a lot, then the next year they're not quite as effective."

A great bullpen can turn a solid team into a pennant winner. The Kansas City Royals last season rode Greg Holland, Wade Davis and Kelvin Herrera to within one victory of a World Series title, encouraging other teams to use a similar approach.

Some of 2014's top relievers earned big bucks in free agency, closer or not. The New York Yankees signed left-hander Andrew Miller to a $36 million, four-year contract. David Robertson ($46 million, four years) and Zach Duke ($15 million, three years) went to the Chicago White Sox, while the Houston Astros added Luke Gregerson ($18.5 million, four years) and Pat Neshek ($12.5, two years).

But past results don't guarantee success going forward. Grant Balfour had 38 saves for Oakland in 2013, then signed with Tampa Bay and lost his closer role before the All-Star break. Jim Johnson reached 50 saves in both 2012 and 2013 for Baltimore, then posted a 7.09 ERA last year for Oakland and Detroit.

"Because there's a lot of variability in performance from one year to the next, and the sample size is not as big as a starting pitcher, you get players that over 50, 60 innings one year look very different than the next year," Astros general manager Jeff Luhnow said.

Last season, 41 major league pitchers had an ERA of 2.50 or lower while working at least 50 innings and making no more than five starts, according to STATS. Those same benchmarks were hit by 35 pitchers in 2013 and 34 in 2012. The problem? Exactly two pitchers made the list all three years — San Diego star Craig Kimbrel while he pitched with Atlanta and Baltimore righty Darren O'Day.

Hardly any reliever can string together several great seasons. That's what made Mariano Rivera so remarkable.

This year, Soria has bounced back from his injury woes and taken over as Detroit's closer. Nathan, on the other hand, is out for the season with a torn ligament in his pitching elbow. Miller is off to an excellent start for the Yankees, but Fernando Rodney — the major league leader in saves last year with 48 — has a 7.36 ERA in the early going for Seattle in 2015.

Dombrowski likened Kansas City's current bullpen to Cincinnati's "Nasty Boys" — the trio of Randy Myers, Rob Dibble and Norm Charlton that helped the Reds to a World Series win in 1990. But despite their amazing single season, the Nasty Boys each had a hard time replicating that success.

It remains to be seen how long Kansas City's bullpen can dominate. Rather than just throwing money at the biggest names, the Royals built their relief corps with foresight and creativity.

Davis was a starter as recently as 2013, but moved to the bullpen toward the end of the season. He went 9-2 with a 1.00 ERA last year and is now serving as interim closer while Holland recovers from a right pectoral strain.

Assistant general manager J.J. Picollo said the Royals began emphasizing the bullpen around 2008.

"We didn't feel like we had the depth in our starting rotation — plus, that was something we could invest in, we could spend money on relievers without blowing out our budget," he said.

Kansas City began looking at its minor leaguers with relief potential in mind.

"When is the right time to take a guy starting and put him in the bullpen? Holland started a little in A-ball, Herrera was in and out of the rotation," Picollo said. "We reached a point with those guys — they're relievers, power relievers. We knew they had good arms."

Since relief success is tough to predict, a quantity-over-quality approach helps. Teams with more organizational depth have an advantage and it's important to be flexible.

Tony La Russa helped popularize the idea of a one-inning closer when he managed the Oakland Athletics in the '80s and early '90s and had Dennis Eckersley. He didn't have similar luxuries when he won World Series titles with St. Louis in 2006 and 2011.

In 2006, closer Jason Isringhausen was injured for the postseason. Adam Wainwright, then a rookie, filled that role, while Josh Kinney and Tyler Johnson — neither of whom had been in the majors at the start of the season — got crucial outs for the Cardinals all through October.

Five years later, La Russa completely overhauled his bullpen during the season. Jason Motte, who didn't have a single save that season before late August, was the postseason closer.

"You can't script a season," said La Russa, who is now in charge of Arizona's front office. "You've got to be ready for whatever."

Teams can be optimistic about their bullpens, but they have to adapt and adjust during the season. Easy for Kansas City a year ago, but the Royals were an exception, not the norm.

No matter who makes the playoffs, chances are some of the most important October pitches will be thrown by relievers who were afterthoughts on opening day.

"The bullpen now is as important as the starting rotation," La Russa said. "You don't have to have an October championship bullpen right away. You've got to have some guys so you can get a win, but it's OK to evaluate and tweak and adjust and move guys around. It's OK. That's why you play six months."

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AP Sports Writers Bob Baum and Dave Skretta contributed to this report.