Blue Jays’ Zach Jackson has potential as MLB reliever


If the Blue Jays are looking to build a bullpen from within, they may want to look to Zach Jackson, who’s got serious potential





The 2016 MLB Draft was the first under the leadership of the Shapiro-Atkins front office. Two years on, we are starting to see just how fruitful that draft was for the Blue Jays. Bo Bichette, their third pick, has performed so well over the last three seasons that he is arguably the best prospect from the entire draft, league-wide—for the record, FanGraphs has him first and Pipeline has him third.


T.J. Zeuch, their first pick, just completed a strong first season at Double-A, finishing tenth in FIP (3.08) among qualified pitchers at the level. He’s looking increasingly like the dependable #4/5 starter he was projected to be. After making some effective adjustments last winter, Cavan Biggio, the team’s sixth pick, dominated Double-A and looks increasingly like he will become a productive major-leaguer.


There are further success stories beyond the team’s three top prospects from the draft. The team’s second-to-last pick, Chavez Young, has shown enough talent in all facets of the game to warrant a 40 FV rating from FanGraphs. The team’s second pick, J.B. Woodman, hasn’t lived up to his potential. Fortunately, he was moved for Aledmys Diaz last offseason, himself moved this offseason for pitching prospect Trent Thornton.


Finally, there’s Zach Jackson. Drafted as a reliever with the team’s fourth pick, he has pitched well in his time as a pro, posting a career 2.60 ERA and 3.34 FIP. His primary pitches are an above-average fastball (50 FV, FanGraphs) and a plus slider (60 FV), with a decent changeup (50 FV) as his third pitch. He’s moved fairly quickly through the system, playing at Short Season-A after the draft, followed by a 2017 season split between Low- and High-A, before spending all of 2018 at Double-A. Throughout his journey, three things have been clear: Jackson strikes out batters by the boatload, generates a lot of weak contact and has a bit of a walk problem.


This past season, Zach Jackson stood out in countless ways among Double-A relievers (all stat references focus on the 179 Double-A pitchers who pitched at least 30 innings and made at least 90% of their appearances in relief). Age-wise, Jackson was young for the level—there were only three qualified relievers in their age-21 or -22 seasons. Jackson was among 25 relievers in their age-23 season, so a total of 151 of his fellow relievers were older than him. He also stood out in terms of workload, with his 62 IP ranking in the 88th percentile.


He displayed an impressive ability to strikeout batters (28.5% K rate, 75th percentile). This is line with his performances at lower levels, where he produced K rates in the low-30s. His strong K rate is well-supported by his 15.5% whiff rate (87th percentile).


He’s also displayed an impressive ability to generate weak contact. He gave up only 0.29 homers per nine innings (80th percentile) and limited opponents to a .200 BABIP (98th percentile). While extreme, it’s worth noting that he has produced similar numbers at lower levels—from rookie-ball to High-A, Jackson gave up only two homers (0.26 HR/9) and produced a better-than-average BABIP (.278).


Moreover, his ability to limit homers and base-hits is reflected in his underlying batted ball stats. At face value, this might not be entirely clear—as a share of the batted balls he surrendered, only 32.3% were grounders (12th percentile) while 53.8% were flies (4th percentile). That said, he limited line drives very well (13.8%, 91st percentile). He also generated a ton of infield flies—31.4% of the flies he conceded were of the infield variety (85th percentile)—which makes his high FB% look a little better—36.9% of his batted balls were outfield fly balls (12th percentile).


Jackson’s biggest issue are all of the walks he concedes—his 19.4% BB rate puts him in the 2nd percentile among qualified relievers. Like his high strikeout rate, his high walk rate has followed him throughout his pro career—he walked 13.1% of the batters he faced from rookie-ball to High-A. The culprit is his poor command, to which FanGraphs has given a current grade of 30 and a future grade of 40.


Jackson’s walk rate last season was high enough to dim the shine of his high strikeout rate—his K/BB (1.47, 10th percentile) and K-BB% (9.1%, 20th percentile) were both well below-average. Even his FIP (3.80) graded out a little below-average (46th percentile), in spite of such strong strikeout and home run rates.


Interestingly, Jackson’s ERA (2.47, 82nd percentile) and xFIP (4.63, 16th percentile) are much lower and much higher than his FIP. His low ERA likely reflects the fact that, while he walked a lot of batters, he didn’t give up many base hits and struck out a ton, allowing him to strand quite a few of them (77.7% LOB%, 80th percentile). His high xFIP reflects the fact that the fly balls he conceded turned into homers far less often than they did for the average pitcher (2.9% HR/FB, 87th percentile).


With such divergent ERA, FIP and xFIP, it’s important to give some thought to which metric is likely to be the best reflection of his true talent. Obviously, as a Jays fan, I’m rooting for it to be his low ERA (I thought it best to make my bias clear). And, indeed, I see good reasons to believe that Jackson’s age-23 season in Double-A true talent lies pretty close to his strong ERA.


For his xFIP to be a fair reflection of his talent, we’d have to assume that his “true” HR/FB is league-average. While possible, it seems unlikely to me. At the lower levels, he produced a 3.2% HR/FB, so his strong 2018 mark doesn’t look like a lucky season. That makes intuitive sense, as he generates so many infield flies that an unusually small share of his fly balls are of the dangerous outfield variety—68.6% of his flies went to the outfield, compared to the level-average of 77%. Moreover, his aforementioned ability to avoid giving up line drives means that he’s going to give up relatively few line drive-home runs, which serve to increase a pitcher’s HR/FB.


There’s also some visual evidence that his low home run rate wasn’t a fluke—his spray chart. Jackson gave up two homers last year, both no-doubters that would have cleared the walls at Kaufmann Stadium. Only three other batted balls would’ve made the warning track. Even if all three cleared the fence, he still would’ve produced a slightly below-average HR/FB (7.1%).



For his ERA to be a fair reflection of his talent, we’d have to assume that his “true” BABIP is in fact around .200 and that his high strand rate is sustainable. While a .200 BABIP is unsustainable—among MLB relief pitchers with 100+ IP over the last three seasons, Carl Edwards Jr.‘s .220 BABIP is the lowest of the bunch—a mark well below .300 seems doable—nearly a third of relievers in this sample ran a sub-.280 BABIP. Does Jackson look like the type of pitcher who can run a low BABIP over the long haul? Well, pitchers who generate few line drives and a lot of pop-ups run lower BABIPs, all else being equal. That certainly sounds like Jackson to me.


His high strand rate is also sustainable, so long as he keeps striking batters out (which he hasn’t had trouble with as a pro). So, if he can keep his BABIP well below .300 and keep up that high strand rate, he can continue running an ERA that’s a lot better than his FIP. With his high walk rate, that will need to be the case for him to achieve his big-league dreams.


Another strong piece of evidence supporting the sustainability of his low ERA is his low wOBA against (.257, 87th percentile). Like his ERA, his wOBA against ranks him among the top twenty percent of Double-A relievers in 2018. wOBA is a very useful stat when assessing pitchers, as it focuses on batter-by-batter results. While it still falls victim to batted ball luck, it overcomes the issue of cluster luck, as it rewards pitchers for preventing productive at-bats and doesn’t concern itself with runners on base and the like. Obviously, pitching well with runners on is useful, so wOBA should be used alongside these other metrics.


The clearest evidence that Zach Jackson has the potential to be a truly elite reliever is his wOBA against in all non-walk plate appearances. In 2018, he faced 263 batters, walking 51 of them. Against the 212 batters he didn’t walk, he surrendered a .147 wOBA, the single-best mark among qualified Double-A relievers. Even modest improvements in his 19.4% walk rate could see him develop into a very dangerous reliever.


His time at the Arizona Fall League produced numbers very much in line with his 2018 performance at Double-A. He struck plenty of guys out (30.9% K rate), generated weak contact (.206 BABIP and no home runs allowed) and gave up too many walks (16.4% BB rate). Overall, he produced a solid 3.44 FIP (76th percentile among pitchers listed on the AFL leaderboard). His ERA (4.05) was only average, but was heavily skewed by one outing that produced half of his AFL earned run total. In that outing, he left the game with one out and the bases loaded (two walks and a single), after which his replacement allowed all three to score. If we exclude this particular outing, Jackson produced a 2.13 ERA (feel free to completely or partially discount this due to cherry picking).


This past season, Zach Jackson displayed a lot of the skill set needed to be an elite reliever. He struck out a ton of batters, gave up few base-hits and didn’t get burned by the long ball. His only weakness is walks, due to his below-average command. My sense is that the good outweighed the bad in 2018, reflected in his strong ERA and wOBA, which makes me think that he could be a useful big-league reliever even if he never completely figures out his command issues. If he does improve his command and improve his 30 grade to the projected 40 grade, that would likely get his walk rate closer to 11%. That is a level at which, alongside his strikeouts and weak contact, he can be a very productive big-league reliever.


With Double-A conquered, I’d expect Jackson to start the 2019 season at Triple-A. If he performs well there, a promotion to the majors is not out of the cards—in their most recent update, FanGraphs has Jackson’s MLB ETA at 2019. Last season, when he was still on Pipeline’s Blue Jays’ Top 30 list, Jackson’s ETA was an even more aggressive 2018. Whichever level he pitches at next season, I’ll be keeping a close eye on his that walk rate of his.







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Jeff Quattrociocchi

I'm an economics professor in the GTA whose lifelong love for the Jays was reignited by that magical August of 2015 and the amazing moments since.