The right-hander has hardly pitched in three years, but on talent alone, manager Buck Showalter believes he could handle being in the majors in 2018. Whether he does so has a complicated precedent with the club and Dylan Bundy.
Castro, 23, has a chance to become the useful starter he was on track to bloom into when the Toronto Blue Jays plucked him from Single-A in 2015 and made him their Opening Day closer, setting off two seasons of tumult and injury that the Orioles managed to settle.
Whether the Orioles have done enough to put him back on that starting track will be one of the biggest challenges of spring training — but perhaps the one with the highest ceiling.
"I know it's a different role coming from being a reliever to a starter, but I'm getting prepared physically and mentally," Castro said Tuesday through pitching instructor Ramón Martínez.
"In the past, I've been through some injuries. I think last year, I was healthy and knew what I could do. I think I learned from that. I became better, got the opportunity. That's the main thing. I'm still going to show what I can do."
Though his experiences have taken him far from his starting roots, the Orioles hope it's not so far that he can't get back to them quickly. The club has recently taken to trying to reverse-engineer relievers into starters for development purposes, most notably with Tanner Scott and Jesus Liranzo at Double-A Bowie in 2017 in an effort to get them more innings and the side work that comes with a rotation spot. But Castro's project is something different considering the only real driving force of it is the club's need for starting pitching.
The Orioles go into spring training with two established starters, and Castro's success in 2017 — he had a 3.53 ERA in 66 1/3 innings — makes him an ideal candidate to fill one of the open spots. So does his young, pliable arm.
"He's got some background in it," Orioles manager Buck Showalter said. "He's had some extended outings, obviously, with us. It's nothing that we can't do in the spring. Unless things drastically change with the makeup of our roster, I think we'd probably start out trying to stretch him out. He's that option for us."
That he's barely 23 helps from not only a developmental standpoint, but also a roster one. Though Castro has been optioned three times — first in 2015 by the Blue Jays, then in 2016 by the Colorado Rockies and in 2017 by the Orioles — he was granted a fourth option because those were used in his first four full professional seasons.
Castro isn't yet married to the major league roster, though the fact that he could fill either a starting or relief role will make him an attractive option, especially earlier in the season as the team's roster is still sorting itself out.
However, none of that lessens the importance of this spring for Castro. The Orioles only got one look at him as a starter last season, and it came at the end of a taxing second half when Castro threw the most relief innings of any pitcher in baseball, 46 2/3.
In his one start, he labored through 3 1/3 innings on Sept. 30 at the Tampa Bay Rays, allowing three runs on six hits while throwing 59 pitches. His longest outing actually came in relief Aug. 3, when he threw six scoreless, one-hit innings.
Castro said Tuesday that the workload didn't affect him or his performance down the stretch, though his acknowledgment that being a starter required a different preparation suggests he'll be more prepared for another innings jump this year.
Absent much other evidence, though, the Oriole can only look at his track record. Showalter likes to point to the difference between a starter and long reliever being the ability to turn over a lineup more than once, but Castro faced just 31 batters a second or third time in 2017 as a reliever.
Despite a live fastball and two secondary pitches, his 5.16 strikeouts per nine innings were the seventh lowest among all major league pitchers with at least 60 innings last season. Yet he also had the ninth-lowest batting average on balls in play among that subset of major league pitchers (.227), indicating he was hard to square up and didn't allow much good contact.
Orioles left-hander Tanner Scott is much more comfortable with the idea of being a starter than he was this time last year, and is likely going to be in that role again as the Orioles refine his impact arm.
None of that jibes with the power reliever profile, but when extrapolated over multiple times through the lineup, when he might mix his pitches more, there's the potential for improvement.
Among those who have pitched alongside him, there's little doubt Castro will succeed. Contrary to his reputation before he came to the Orioles, he's become a favorite among those in the clubhouse, and a friend to both the English-speaking players and those from Latin America. The blond braids he wore to minicamp this week left the coaching staff delightedly amused, and his comfort in his surroundings bodes well for what will be asked of him this spring.
"He came up as a starter, became a reliever slash closer, setup guy, long reliever," Scott said. "I don't think it's going to affect him at all. He's Castro. ... With the success he had in throwing anywhere from one to, what, six innings? That's pretty good.”
In a spring that will be heavily focused on figuring out who will pitch alongside Dylan Bundy and Kevin Gausman in the starting rotation, the Orioles will pay close attention to whether Castro fits there.