Every year, one of the most important aspects of a winning season is one of the least predictable: How’s that bullpen going to work out? It might not be that hard to project if a team will have a strong offense or a solid rotation, yet the ever-changing members of a bullpen seem to be the most inscrutable part of a team – in part because the names at the beginning of the season so often aren’t the same names that are there at the end.

It’s not just because of injuries or inconsistency, either. It’s also because of breakout relievers. It’s become a cottage industry for winning teams to do this, because each year, the most advanced of them find pitchers who have stopped being successful – or never really were in the first place – and with a tweak here and a new pitch there, turn them into valuable arms. Think the Dodgers with Evan Phillips and Ryan Brasier; think the Rays with Robert Stephenson and Zack Littell; think the Phillies with Jeff Hoffman; think the Yankees with Clay Holmes; think the Mariners with Paul Sewald.

Of course, you often can’t see these things coming. Remember Yennier Cano, who had an 11.50 ERA in 2022 and then was essentially perfect for the first two months of 2024 on the way to a breakout season? He got there by changing his release point and adding seam-shifted wake to his sinker. Tanner Scott had spent years not throwing enough strikes, and then he … did. Sometimes, at least from the outside, it's not quite so clear what's about to happen.

That doesn't mean we can't have fun trying to guess, though. So who will be 2024’s breakout relievers? These names are among our favorites.

Gregory Santos, RHP, Mariners

Santos is a 24-year-old reliever who has already been through three organizations, and he’s now on to his fourth. He’s got a career 4.00 ERA, and he was DFA’d by the Giants as recently as December 2022. His 2023 season ended early due to elbow inflammation. Quite the sales job, isn’t it?

It should tell you something, however, that Seattle was willing to trade two of its top 25 prospects and a Competitive Balance Round pick to acquire him, as it did last week, because this deal presumably means that the Mariners are not too worried about his elbow. Instead, they’re buying into his elite velocity, and it’s not just the 99 mph fastball; it’s the 91.4 mph slider that’s the fifth-hardest slider of any regular pitcher – and the second-most valuable slider, at that.

It’s not that Santos didn’t have the slider in his brief Major League looks before 2023, though it was a little more cutter-ish last year. It’s that he traded in his hard-but-not-that-effective four-seamer for a sinker that had more than twice as much horizontal break, thanks to considerable seam-shifted wake effects, and looks like, well, this:

It also plays better with the similarly horizontal slider, giving it a better mirroring effect. It’s hard, too, not to be impressed by the words of Tigers broadcaster Jason Benetti, who was with the White Sox last year and spoke to Seattle Sports about his first-hand looks.

“You’re talking about all the metrics that suggest he’s going to have a breakout year,” said Benetti, “and I wouldn’t disagree. I mean, he just completed his first season with a two-seamer,” also adding that White Sox pitching coach Ethan Katz “felt like, specifically, that the four-seamer just wasn’t doing as much, [and] that the sinker paired better with the slider, as we’ve seen in baseball lore. He had very good success with his sliders, specifically, off of the sinker.”

The team that helped fellow hard-throwing slider-owners like Matt Brash and Andrés Muñoz improve might just be a perfect fit.

Fernando Cruz, RHP, Reds

Cruz is a nearly 34-year-old pitcher coming off a 4.91 ERA. We said under the radar, right? He was drafted as an infielder by the Royals back in 2007 and converted to pitching in 2011, then spent most of the decade in Mexico or independent ball before finally getting to the Majors at 32 with the Reds in 2022. Last year, he had a 4.91 ERA and missed time in May with a shoulder strain. If that sounds like Santos, well, at least Santos is a decade younger.

Cruz doesn't exactly have the profile of a breakout candidate, and he wouldn’t be one if not for that splitter, which is one of the deadliest pitches in the game. Last year, more than 1,150 combinations of pitcher and pitch type got at least 100 swings against them, and only three of those had a higher whiff rate than the 57% that Cruz got on his splitter. (They’re three Big Time Pitches, too – Felix Bautista’s splitter, Robert Stephenson’s new cutter and Kodai Senga’s Ghost Fork.)

So Cruz has a great pitch, and the splitter is a pitch that seems like it’s about to have a moment. (In a forthcoming update to Statcast's capability to track the path of the bat, Cruz's splitter is going to end up atop an extremely interesting leaderboard, too.)

But he already had that pitch last year. So where did the 4.91 ERA come from? In part, it was a dreadful final outing of the year – allowing five earned runs in one inning, as he did on Sept. 30, won’t help anyone’s ERA – but it’s also hard not to notice the excellent 2.91 Fielding Independent Pitching and solid 3.45 expected ERA.

In addition, the 2024 Reds defense can’t possibly be as porous as the 2023 version was (29th in Outs Above Average), and the gap between Cruz’s 4.91 ERA and 2.91 FIP was the second-largest among relievers in baseball, behind only Aaron Bummer, himself seen as a rebound candidate after Atlanta looked past his 6.79 ERA.

Dauri Moreta, RHP, Pirates

If you know ball, you might already think Moreta broke out, but a 3.72 ERA and one save won’t exactly pierce the public consciousness, so go with us here. The success that Moreta had in 2023 was mostly because of the pitch that he calls a slider, a bizarre offering that was described by his coaches and catchers as something more of a screwball.

Whatever it is, he threw it a lot last year, doubling his usage of the pitch from 24% with the Reds in 2022 to 65% with the Pirates. That helped him jump his strikeout rate from 24% to 32%, and 58 of his 75 strikeouts came on ... whatever that pitch is.

That was last year, though. What makes us think that there’s maybe more in there for further improvement? That might depend on what you mean by improvement, really, because if it’s just about ERA, then a 3.02 xERA and a 2.93 FIP might tell you that he already had a stellar year. (The ERA nearing 4.00 came mostly from a few bad outings; nearly half of his earned runs on the year came in just four games.)

But there’s also what we saw late in the year, when Moreta returned from a brief trip to the injured list and Minor Leagues. In five outings over the last two weeks of the season, he threw the pitch nearly 80% of the time … and struck out 10 of the 23 batters he faced over seven perfect innings.

"If you have one weird or great pitch, throw it a whole lot" sums up a great deal about pitching strategy over the last decade. How much will Moreta throw his backwards slider in 2024? The answer might define how effective he can be.

Trevor Megill, RHP, Brewers

Megill is somewhat similar to Moreta, in that he garnered some notice in 2023, yet also has a career 5.23 ERA and zero career saves, so we’re assuming the larger public knows little about him. That’s in part because it’s taken a long time to get here; Megill was a seventh-round pick by the Padres in 2015, a Rule 5 choice by the Cubs in 2019, a waiver claim by the Twins in 2021, and finally found his way to Milwaukee in a trade you absolutely did not notice on April 30, when the Twins received a player to be named later.

The Brewers assigned Megill to Triple-A after the trade and sent him back there twice more over the summer, so this isn’t exactly an instant win for Milwaukee. On the other hand, Megill had a 6.03 ERA and 25% strikeout rate in his time with the Cubs and Twins; he had a 3.63 ERA and a 35% strikeout rate in his first year with the Brewers. When we looked at 2024 Steamer projections from relievers, it was hard to miss Megill popping up in the top 15.

What changed? For one thing, the velocity jumped from good to elite. As a Cub, Megill was throwing 96.4 mph. As a Brewer, he was throwing 99.1 mph, touching triple digits 98 different times.

Megill credited that velocity improvement to his time with Minnesota, saying in 2022 that “the Twins have done a great job of really just making me understand myself and my body and everything. So there's a big factor that they put into the equation.” He credited learning the curveball (now thrown at 85.8 mph, one of the hardest curves in the game) that replaced his old slider to his time with the Cubs, and that pitch allowed only two extra-base hits last year.

It takes a village, as they say. The Brewers deserve credit for identifying Megill as a pitcher to acquire, but much of this value was in motion before that.

James McArthur, RHP, Royals

Another day, another high ERA (4.63) from a pitcher new to his team (McArthur was traded to the Royals in May after the Phillies designated him for assignment) that may not really tell you enough about the player. That worked out OK for Philadelphia, because the roster spot went to Jeff Hoffman, who was a big part of their bullpen down the stretch, but it might have worked out well for Kansas City, too.

That’s because McArthur, who had been a starter in the Phillies organization, became a reliever when he reported to Triple-A Omaha. His Major League debut on June 28 was humbling; he allowed seven earned runs in a single inning to Cleveland – and there’s the high ERA for you – but when he returned from the Minors in August, he got into 17 games and he was tremendous, striking out 23 against a single walk, earning a 2.01 ERA and 2.05 FIP in 22 1/3 innings.

In McArthur’s case, it was a little about a velocity jump, as his sinker went from 93.2 mph with Philadelphia’s Triple-A club to 94.1 mph with the Royals, as you might expect from a starter going to relief. But it’s also about Kansas City instructing him to use his curveball as his primary pitch, because after using it 21% of the time with Lehigh Valley, that went up to 45% of the time with Omaha and stuck at 39% with the Royals.

The Royals, for years, have struggled to develop pitchers into Major League performers; look no further than how much their vaunted 2018 Draft class of pitchers have all struggled mightily to stick in the bigs. But there are some signs of progress there, most notably how Cole Ragans looked like a top-level ace after the Royals got him from Texas last summer. McArthur might be on the precipice of that jump, as well.

(We might have included undrafted Royals teammate John McMillon here also, given his elite velocity and impressive brief look last summer, especially after he finished second in the affiliated Minors in strikeout-minus-walk ratio, but questions about his control and that he missed the final month of the season with a forearm strain led us to prefer McArthur.)

Tucker Davidson, LHP, Orioles

For our final name, we’ll admit we’re digging extremely deep here, because Davidson has a career 5.98 ERA for three teams and actually passed through waivers unclaimed when Baltimore (which had just claimed him from Kansas City) outrighted him to the Minors in November. It’s more likely than not that his season starts against the Durham Bulls (with Triple-A Norfolk) than it does against the Los Angeles Angels (with the Orioles).

Consider this more of a “tuck it away and see what the Orioles' pitching lab can do” for now, mostly because of the splitter that Davidson flashed late last year with the Royals. He unleashed it 63 times, allowing only two hits, and earning 20 misses on the 38 swings it induced. Those are extremely small numbers for a pitch, to be clear, but the ensuing 53% whiff rate would have been third behind Bautista and Cruz, among those with a minimum of 25 swings. It’s probably nothing. But it’s the Orioles, too. They helped Cano improve last year. Is Davidson next?