The Conversion Arm Compendium

Every year, hapless hitters with premium arm strength get moved to minor league mounds. With the help of Sean Dolinar, who combed the last few years of stats to scrounge up a more comprehensive list of converts than I was otherwise able to remember off the top of my head, I assembled the list below of former position players who are now prospects of note as hurlers. This is not a comprehensive survey of every recent conversion arm in the minors. Instead, these are the pitchers I think are interesting enough to include on an offseason list in some capacity.

Conversion arms who pan out typically put it together quickly. For example, it only took Kenley Jansen about a year after he first toed an affiliate’s rubber to reach Dodger Stadium. He likely threw during 2009 Extended Spring Training, then spent the back half of the summer at Hi-A before making a Fall League appearance. He breezed through Hi- and Double-A the following year, and was in Los Angeles by late July of 2010. Jason Motte started his conversion in 2006 and got his first big league cup of coffee in September of 2008. Joe Nathan’s first pro innings came in 1997; he was first called up to the majors early in 1999. Sean Doolittle threw just 26 minor league innings before the A’s brought him up. (Conversely, Alexi Ogando and Carlos Marmol each took about three years after moving moundward to become big leaguers.)

Who in the minors might be next to have impact, big or small, on a big league pitching staff? Here are some candidates. All of the 35+ FV and above players are now on THE BOARD, if they weren’t already.

Javy Guerra, San Diego Padres (40 FV, Org Rank: 31):
I talked at length about Guerra in the Padres list, back when he was sitting in the mid-to-upper-90s during bi-weekly relief outings in Extended. He missed roughly a month with injury, but he owns a 23-to-5 strikeout-to-walk ratio in 16 innings since his return. He’s 23 and wrapping his year at Double-A.

This is the most graceful, fluid athlete on the list, the one with the best present stuff, and best long-term body projection, he’s had statistical success and already reached the upper-minors despite having thrown just 20 innings. If past success stories are our guide, this is what it looks like when conversion guys have big ceiling. Guerra may play a significant role in the 2020 Padres bullpen, and because he’s already on the 40-man, he may even see some September time.

Eric Mariñez, Oakland Athletics (35+ FV, Org Rank: 35):
Mariñez was a promising position player prospect who just never developed a competent bat. He moved to the mound late in 2018 and looked so good during instructs — 93-95, up to 97, with surprising changeup feel and high curveball spin — that I thought he was a Rule 5 dark horse even though he’d barely thrown a pro inning. Now that he has a year of productive relief work on his resume, he’ll likely either be added to the 40-man or be in teams’ Rule 5 mix this winter.

Elvis Alvarado, Seattle Mariners (35+ FV, Org Rank: 27):
Acquired from Washington as part of the Roenis Elias and Hunter Strickland trade return, Alvarado is a 20-year-old with 40 career innings to his name after ditching hitting in 2018. He throws really hard — up to 100 during Extended, to 98 during his tenure with Seattle, sitting 95-97 — and his strike throwing has improved throughout this year. His breaking ball is still gestating, but Alvarado has feel for locating it already. He’s a long-term developmental project who’d probably be a fourth round type of prospect were he a draft-eligible junior college arm.

Kurt Hoekstra, Atlanta Braves (35+ FV, Org Rank: 29):
If anyone on this list raises alarms for how quickly they’ve moved up the minor league ladder, it’s Hoekstra, who made some emergency relief appearances at Triple-A in July, before settling in at Double-A Mississippi, where he has been dominant for the last month. Hoekstra was a high school shortstop and closer who later played second base at Western Michigan, then tumbled down the defensive spectrum in pro ball. He threw a few innings during blowouts in 2018, then became a full time pitcher in 2019.

Hoekstra is not a comfortable at-bat for right-handed hitters, who often bail on curveballs that appear headed for their shoulders or elbows, but instead dip and land in the zone for strikes. He sits 91-94 and has touched 96. He needs a better way to deal with lefties and he has below-average control, but this guy only has 60 career innings so that may still be coming, even though he’s 26.

Eduardo Herrera, Arizona Diamondbacks (35+ FV, Org Rank: 42):
A converted infielder who moved to the mound this year, all 11 of the outs accrued by Herrera in his 3.2 innings of AZL work were strikeouts. Then he had a wild 18 inning at Hillsboro as their closer. He mostly just chucks fastballs in the 94-97 range past low-level hitters right now. He needs a better breaking ball and fastball command, but Herrera is only 19.

Yoan Aybar, Boston Red Sox (35+ FV, Org Rank: 32):
Aybar has premium lefty arm strength — he sits about 96 and touches 98 — but he’s made little headway with fastball control in his two seasons removed from the outfield. Just 42% of his fastballs were in the strike zone this year. He was passed over in last year’s Rule 5.

Francys Peguero, Washington Nationals (35 FV):
A converted shortstop, Peguero has been pitching, as a starter, since 2015. He’s had a pretty fringy four-pitch mix but since finally moving to the bullpen in late-July, he’s got a 21-to-6 strikeout to walk ratio in 17 innings. My notes have him 89-93 before the move to relief but I don’t have an updated report since that time, so perhaps he’s throwing harder in shorter outings.

Brandon Hughes, Chicago Cubs (35 FV):
Hughes threw all of six innings as a freshman at Michigan State and had been a full-time outfielder before his conversion. He’s thrown 30 good innings across the AZL, Northwest League and, for the last few weeks of the season, South Bend. He turns 24 in December but Hughes already has a fair, three-pitch mix — 90-94, a delivery that’s tough on lefties, an average changeup, and a fringe breaking ball — and he’s throwing a lot of strikes. He’s 2020 Rule 5 eligible.

Jesus Tona, San Francisco Giants (35 FV):
2019 was Tona’s second full year of pitching after he saw time at catcher and second base in ‘16-’17. He’s a squat, 23-year-old righty who spent most of this year as Low-A Augusta’s closer, before heading west to Hi-A San Jose in August. He sits 90-93 and touches 94 with some arm-side tail, and hitters seem to have a tough time picking up the ball out of his hand. His changeup has similar, more significant, but also more apparent, movement. He amassed 72 strikeouts in 57 innings this year, generating a 15.% swinging strike rate overall, with an 18% whiff rate on his fastball. He’s Rule 5 eligible this offseason and while he’s produced fairly interesting results, his stuff is short of what is typically selected in the Rule 5.

Isrreal De La Cruz, Detroit Tigers (35 FV):
De La Cruz is 22 and has thrown about 12 pro innings, all in the GCL. He’s at the back of this list, developmentally. But he’ll touch 94 and has some fastball components conducive to vertical movement, including a lot of spin — about 2450 rpm on average — and a favorable spin axis.

Elvis Escobar, Pittsburgh Pirates (35 FV):
A 24-year-old lefty up to 95 with below-average secondary stuff, Escobar has been pitching for a year and a half but lost a chunk of that due to a spring shoulder issue. He was pushed to Double-A in late-August despite having thrown just 10 innings all year.

Jacob Bosiokovic, Colorado Rockies (35 FV):
Bosiokovic only ever played outfield at Ohio State (where he had a Tommy John the summer after his sophomore year), but he pitched in high school. He’s struck out a batter per inning as a 25-year-old at Low-A Asheville working 92-94 and a 40 slider to his glove side.

We hoped you liked reading The Conversion Arm Compendium by Eric Longenhagen!

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Eric Longenhagen is from Catasauqua, PA and currently lives in Tempe, AZ. He spent four years working for the Phillies Triple-A affiliate, two with Baseball Info Solutions and two contributing to prospect coverage at ESPN.com. Previous work can also be found at Sports On Earth, CrashburnAlley and Prospect Insider.

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Question re: Mariñez: Since this season will be his seventh overall in the minors, if he isn’t added to the 40-man, isn’t he a minor league free agent per Major League Rule 3(b)?