The left-hander won High-A Northwest Pitcher of the Year despite being promoted to Double-A mid-season
8. Sean Sullivan (653 points, 28 ballots)
Sullivan was Colorado’s second-round pick in 2023 (46th overall) as the number two starter at Wake Forest behind Reds first-round pick Rhett Lowder. In 2023, he threw 69 2⁄3 innings in 17 games (10 starts) with a 2.45 ERA, 0.92 WHIP (fourth-best in Division 1), a 14.3 K/9 rate (second-best), and a 2.7 BB/9 rate as the Demon Deacons made it all the way to the College World Series semi-finals.
Sullivan rode those numbers to a $1.7 million bonus from the Rockies that was about $187k below slot. The 6’4” lefty relies mostly on his fastball which, despite velocity that is usually only in the 88-92 MPH range, is a plus pitch due to a low, wide release angle that gives it great carry up in the zone. He pairs that offering with a slider and change-up, but the fastball is the main separator.
Rockies brass were impressed in a post-draft MLB.com article:
“[Sullivan] struck out guys,” general manager Bill Schmidt said. “Hitters had problems picking up his stuff. He’s left-handed with a very quality slider and angle to his fastball.” Sullivan, whose motion resembles that of Kyle Freeland, came highly recommended from the Rockies’ analytics department.
After a four inning cameo in 2023 in which Sullivan allowed no hits, one walk, and struck out 10 batters, it was clear that a High-A assignment would be needed to challenge him. In Spokane in 2024, where Sullivan was 2.1 years younger than league average, the statistical dominance continued. In 14 starts with Spokane, Sullivan threw 83 1⁄3 innings with a 2.13 ERA, 0.84 WHIP, 10.9 K/9 rate, and a minuscule 0.9 BB/9 rate (including a nine-inning complete game). Sullivan was clearly in cruise mode in July, when he went just five innings per start in three starts (so 15 IP in total), but allowed just one run on eight hits with two walks and 25 strikeouts (each outing was fewer than 75 pitches).
The Rockies finally showed mercy on Northwest League hitters with a promotion of Sullivan in August to Double-A Hartford, where Sullivan was 3.5 years younger than league average. Sullivan maintained his excellent run prevention against Eastern League hitters, throwing 32 innings across seven starts with a 1.97 ERA. Sullivan wasn’t nearly as dominant, striking out only less than a batter per inning (6.8 K/9) and walking a few more (2.0 BB/9) en route to a 4.37 xFIP.
Still, a 1.09 WHIP with that ERA is a strong achievement for one of the younger pitchers in Double-A (the batter was older than him 80% of the time). Interestingly, the lefty was death against right-handers (.484 OPS allowed) but less effective against lefty batters (.790 OPS allowed) in a smaller sample. Despite the mid-season promotion, Sullivan did enough in the High-A Northwest League to receive their Pitcher of the Year award.
Here’s some video of Sullivan striking out a bunch of dudes in 2024 (mostly in Spokane), many of them swinging:
Keith Law of the Athletic ranked Sullivan 7th in the system earlier this month:
Sullivan comes from a lower slot and throws an invisiball, often just 90-93 but from such a low angle and with so much carry that hitters miss it — and they’ll chase it right out of the zone to an unusual extent. He turns over a 55 changeup despite the lower slot, while his slider is fringy, getting some boost because of the angle at which he releases it so it sweeps down and away from lefties. He threw 83 1/3 innings for High-A Spokane and walked eight; he was horribly wild in Double A, though, as in 32 innings there he walked … seven. Unacceptable, really. Double-A hitters did make more contact on his secondary stuff, which is one thing to watch. Another is his flyball rate, 44 percent in High A and then 51 percent in Double A, just because, you know, (whispers) Coors Field. I really like him, though, because he succeeds without huge velocity or some pitch cooked up in a lab from Sudafed, iodine crystals, and the bones of former Rockies pitchers.
John Trupin of Baseball Prospectus ranked Sullivan 10th in their recent system ranking with a 50 OFP:
While Sullivan did allow his first professional earned runs this year, a 2.11 ERA in 115 ⅓ frames between High-A Spokane and Double-A Hartford is no mean feat. That’s all the truer given that Sullivan is scarcely topping out at 90 mph in most of his outings. Watching Sullivan, however, explains a great deal. The 6-foot-4 southpaw leans heavily to his arm side as he deals, releasing the ball beyond the mid-point of the left-handed batter’s box at a low 3/4ths angle that is more accurately a high sidearm. The VAA created is subsequently disorienting for hitters, and indeed batters were flummoxed by Sullivan all year, swinging under his four-seam despite its meager velocity. That set up Sullivan’s sweeping slider, a devastating pitch which plays up off Sullivan’s angle and his plus command. Though his changeup is more sparingly utilized, the fastball-slider combination is effective enough to efficiently carve through lineups before they can get too comfortable. The margin for error is so thin for a pitcher with such little velo, but Sullivan’s results continue to outpace the radar gun. His heater and slider are effective enough that a bullpen role will be quite reasonable if he does ultimately hit a wall against big-league bats. He’s close enough to make Albuquerque this year, and polished enough to get a crack in Denver if his schtick keeps playing.
Many command and control arms with bat-missing stuff in the low minors can’t keep it up in the bigs. Sullivan already saw his swings and misses dip at Double-A, but he continued getting awkward swings and weak contact. This either works or it doesn’t, but Sullivan seems unlikely to change it up significantly.
Eric Longenhagen of Fangraphs was less of a believer last month due to the low fastball velocity, grading Sullivan as a 40 FV player (with a multi-inning relief role), 25th in the org, albeit with a 70 future command grade:
Sullivan is an ultra-deceptive cross-bodied lefty with 20-grade velocity. He’s done well in the mid-minors despite sitting 87 mph because of a mix of deception, command, and quality secondary stuff. Sullivan’s stride direction takes him toward the first base line at an extreme angle, and this, in concert with his low three-quarters slot, takes hitters a few looks to get comfortable with. Sullivan’s fastball averaged 90 mph in 2023 and 87 mph in 2024, but he still managed a 2.11 ERA, 125 strikeouts and just 15 walks across 115.1 innings, good for a microscopic 3.4% walk rate. In college, Sullivan’s repertoire was fastball-heavy in the extreme at about 75% usage. His usage is still really high (67% in 2024), but he’s been branching out in pro ball and his changeup generated huge rates of chase and miss last year. His slider plays by virtue of Sullivan’s odd release. It’s fair to be skeptical that this will actually work in a starting pitcher capacity — asking a guy who sits 87 to navigate a big league lineup three times feels like too much — but in short relief bursts where hitters have no time to adjust to Sullivan’s funk, he should be fine.
Sullivan was ranked 123rd overall in the 2023 draft by MLB.com, well below where the Rockies took him, but that looks woefully inaccurate now. He slotted 10th in the system as a 45 FV player during the 2024 season thanks to 60 fastball and 55 control grades:
Sullivan is an outlier in a number of ways, not just for being the antithesis of how the Rockies have drafted pitching. Even though his fastball averaged just a tick over 90 mph last year (though he could touch 95 mph), he threw it 75 percent of the time with Wake. Because of his lower slot and wide angle that leads to huge carry up in the zone, it doesn’t get hit even when hitters know it’s coming (36 percent miss rate on the pitch in 2023). Right now, Sullivan’s best secondary offering is his changeup, which can also miss bats and is an easy average pitch. He has a tendency to change the slot on his slider, hurting its consistency, but the Rockies think it could improve as he works on pitch design with pro instruction.
Sullivan does an excellent job of filling up the strike zone despite his unusual mechanics, and he tunnels his mix well to make him even harder to hit. It remains to be seen if this will work in Colorado, but he’s the kind of pitcher you don’t try to change. Seeing how his stuff plays in his first full season of pro ball will be an interesting storyline in the Rockies’ system.
Between Chase Dollander, Sullivan, and Cole Carrigg, Colorado’s first three picks of the 2023 draft all look like big hits so far. Sullivan’s college dominance with his fastball has carried over to the minor leagues in a big way, even up to Double-A, as he races Dollander to be the first from the draft class to make it to the Show. I think Dollander is a good bet to win that race, but it’s still notable that Sullivan could be a MLB contributor as soon as this year.
The Rockies might assign Sullivan back to Hartford to start this year, but I can’t imagine he’ll stay there long, with a summer Coors Field debut in the cards so long as his health cooperates and if his fastball bedevils upper minor league hitters. I ranked Sullivan seventh on my list as a 45 FV player because of the pedigree, statistical dominance, and because I want the Rockies to continue to think more out of the box with their pitching draftees.