Regarding Cooperstown, you've probably heard of the existence of small-Hall people -- those people who prefer that the Hall of Fame include only the very best of the very best. Myself, I'm a big-Hall person. There have been a lot of great players; induct 'em all!
Just as I'm a big-Hall person, I'm a big-ace person. This is its own active debate. Some believe that, in any season, there are maybe five, maybe 10 aces. I think of aces as existing in greater numbers than that, though. To me, an ace starter is someone good enough to be the No. 1 starter in an average rotation. Some teams might be lucky enough to have multiple guys who match that description. Other teams are more like the Chicago White Sox. I just want to make sure you understand where I'm coming from.
I think there should be right around 30 aces. On many of them, we'll all agree. Jacob deGrom is an ace. Chris Sale is an ace, obviously; so is Corey Kluber. You're already familiar with the cream of the crop. My list might include a few more names than yours, and that's fine. Another crucial point to understand is that the list is always changing. Some players decline or get hurt, while other players improve. That's the story of baseball. That's certainly the story of pitching.
So let's turn our eyes to the future. We can already say who's likely to remain an ace. That may not be very interesting. Those are players we know are already great. It's more interesting to talk about newer developments. Today, let's get into the starting pitchers on the verge of becoming aces in 2019 -- if not by your definition of aces, then by mine. Guys capable of being No. 1 starters, and guys who still have a lot in the tank.
There's no guarantee for anybody, of course. Careers are seldom perfectly linear and predictable. But I'm a fan of all of the 10 pitchers below, and I'll try to quickly explain to you why. In alphabetical order, here are my aces in the making:
Luis Castillo, Cincinnati Reds: I felt similarly about Castillo a year ago. In a sense, maybe it's disappointing he didn't already break out last year. But for as much work as the Reds have done to improve by adding players from outside the organization, arguably the most important Red is someone they already had in-house.
Castillo is a 26-year-old with a 96 mph fastball. It would be easy enough to say he had a big second half. In truth, he had a big final five months after a frustrating April. From May 2 on, Castillo struck out a batter an inning, and he had the same xFIP as Noah Syndergaard. An excellent changeup gives him what he needs against left-handed hitters. It's simply going to come down to fastball command. If Castillo can tap back into the sinker he threw in 2017, it'll be lights out for opponents. They don't make starters much more intriguing.
Mike Clevinger, Cleveland Indians: It's easy to lose track of Clevinger, given his presence in the Tribe's rotation behind Kluber, Carlos Carrasco and Trevor Bauer. But that shouldn't be held as a point against Clevinger himself. He started to come on strong in 2017, but he reached a new level down the stretch in 2018. Over the final three months, Clevinger ranked in the 88th percentile in strikeout rate among starters. Even more importantly, between the past two years, Clevinger's strike rate improved by 4 percentage points, which is a meaningful shift. He's not hurting for velocity, he throws four pitches, and more than ever before he has learned how to spot them. The Indians might well feature a four-ace rotation. And I'm also a big fan of Shane Bieber.
Jack Flaherty, St. Louis Cardinals: I'm not sure how it is that Flaherty got so lost in award consideration. As Ronald Acuna Jr. ran away with the National League Rookie of the Year, Flaherty finished fifth, with just two points. That despite a 30 percent strikeout rate and a 3.34 ERA. Flaherty might not be the world's most consistent strike thrower, but over six individual months last season, his worst strikeout rate was 26 percent. Flaherty can punch out both lefties and righties, and all that's really missing is an improvement with runners on base. Add some consistency there, and the Cardinals will have a boost as they push for the NL Central title.
Andrew Heaney, Los Angeles Angels: Heaney was quietly the one stable contributor in the Angels' rotation last year, making 30 starts and throwing 180 innings. And there were a few interesting parts to that. First, Heaney threw with career-best velocity. Second, Heaney delivered a career-best strike rate. And third, Heaney enjoyed a career-best success rate against lefties. He trimmed his walk rate as the season wore on, and Heaney had never before thrown such an effective changeup. With another year now between him and Tommy John surgery, he should be ready and able to shoulder the load as the best version of himself.
Jakob Junis, Kansas City Royals: Somewhere around a year ago, I told a front-office friend of mine to see about trading for Junis. It was only a half-serious prompt, yet he responded that he'd tried but the Royals knew what they had. Junis didn't exactly light the world on fire in 2018, but he showed enough to keep me interested. He leaned ever more heavily on his Kluber-esque, sweeping breaking ball, and after allowing 2.1 homers per nine innings in the first half, he dropped that below 1.0 in the second half. He also improved his walk rate and kept more balls on the ground. Junis probably won't be the next Kluber, because "the next Kluber" sets an impossibly high bar, but many of the elements are present for a similar breakthrough. All it will take is a slight improvement in command.
German Marquez, Colorado Rockies: I think Marquez tops just about every list of 2019 breakout candidates. That's because you could argue he's already broken out. He just improved his strikeout rate by 34 percent, season to season. Half to half, he improved his strikeout rate by 44 percent. In the second half of last season, 111 starters threw at least 50 innings; Marquez ranked fifth in strikeout rate, 12th in park-adjusted ERA and fourth in park-adjusted FIP. Marquez has long had velocity, but the real difference-maker for him was unveiling a new slider that quickly became one of the most effective sliders anywhere. All that's left is to pitch the same way over a full season. Marquez, you could say, has done the hardest work. Now he just needs to be seen for it.
James Paxton, New York Yankees: This is a tricky case. I think, in terms of his on-field performance, Paxton already has done enough breaking out. He's already done enough to establish himself as one of the more dominant starting pitchers in either league. It doesn't really matter what statistic you look at -- Paxton is great by strikes, strikeouts, walks, ERA and so on. The reason he's not more widely recognized as an ace is because he's been hurt. Paxton has yet to throw even 170 innings in a major league season. It is, without doubt, important to stay healthy. And I'm not prepared to believe Paxton is injury-prone. In 2016, he was placed on the disabled list because of a comebacker. Another comebacker sent him to the DL in 2018. He also experienced minor back soreness. It's true that, in 2017, Paxton went to the DL with two different strains, and that's not good. But his specific injuries don't seem to be chronic. With health in 2019, Paxton could be the best starter in a Yankees uniform. He has all of the talent he needs.
Nick Pivetta, Philadelphia Phillies: Like some of the pitchers above, Pivetta doesn't need to do all that much more than he already has. The key for Pivetta going forward will be getting his ERA to match up with his peripherals. Last year, out of 128 starters with at least 100 innings, Pivetta ranked 109th by park-adjusted ERA. He ranked 45th by park-adjusted FIP, and he ranked 25th by park-adjusted xFIP. By Statcast's expected wOBA metric, he ranked within the upper quarter of starters, with the biggest problem having been the bad Phillies defense behind him. With better play in the field behind him, Pivetta will emerge, even though by many metrics he already has emerged. It just understandably takes a while before the watching public might catch on.
Jameson Taillon, Pittsburgh Pirates: Pretty obviously, if the Pirates are going to compete in 2019, they'll be relying on players they already had to step forward. Taillon is an excellent candidate to do exactly that, having been another one of those guys with a midseason improvement. Taillon threw his first slider of the season on May 11. He first featured it heavily in his repertoire on May 27. Comparing his time before and after, he trimmed his ERA by almost two runs, he trimmed his FIP by almost one run and he trimmed his xFIP by a half-run. The slider gave Taillon a complete four-pitch repertoire, and this is a guy who already got his fastball into the mid-90s. Once more, all that's left is a full season of consistency. Taillon didn't quite flip the switch as dramatically as Marquez did last year, but that doesn't mean they aren't following similar paths.
Zack Wheeler, New York Mets: Wheeler has sometimes come off like the forgotten effective part of the Mets' starting rotation. The fact that he missed two seasons doesn't help, but that chapter of his career is now long behind him. In 2018, Wheeler threw with career-best velocity, and he was at his best in the second half, posting a 1.68 ERA. The big adjustment for Wheeler was that he ditched his two-seam fastball in favor of leaning on his four-seamer. He also started using that four-seamer to bust both righties and lefties inside. According to the final numbers at FanGraphs, Wheeler wound up with baseball's fifth-most valuable four-seam fastball, behind just Justin Verlander, Max Scherzer, Gerrit Cole and deGrom. It's flattering company, and Wheeler belongs. Moving forward, he's a huge reason why the Mets are setting their sights on a postseason berth.