Below are some sleeper prospect candidates who might have an impact on the 2018 MLB season, or maybe even your fantasy team. "Sleeper" is a relative term that means something different to each fan, so this is my best guess at names that ESPN Insiders probably don't already know much about. All of the players listed below are currently on their parent club's 40-man roster and still have rookie eligibility.
National League
Arizona Diamondbacks: RHP Yoshihisa Hirano. This 33-year-old righty has been a dominant reliever in Japan since moving to the Orix Buffaloes' bullpen in 2010. He tees up hitters with a low-90s fastball and then puts them away with an above-average, low-80s splitter. Arizona's bullpen situation might even allow Hirano to stumble into some save opportunities if they don't add anyone before the spring.
Atlanta Braves: LHP Adam McCreery. This assumes you're already aware of oft-injured lefty A.J. Minter, who threw some big league innings last year and has closer stuff. McCreery is the deeper sleeper, acquired from Anaheim in a May 2016 trade for Jhoulys Chacin. He was a 23-year-old who hadn't pitched above rookie ball at that time, but 18 months later he's on a 40-man roster. He has a relatively innocuous fastball/curveball combo, but he's 6-foot-8 and it takes hitters a few pitches just to get comfortable against him because his pitches look like they're falling from the sky. He could get a chance to make a bullpen impact for Atlanta this year.
Chicago Cubs: RHP Alec Mills. Mills will touch 95 mph with his fastball, but sits about 88-92 to go with plus command of two solid breaking balls and an above-average changeup. He's a pitcher's pitcher who throws whatever he wants, wherever he wants, in any count, but he has to because he has a 40 fastball.
Cincinnati Reds: RHP Jose Lopez. Lopez's stuff spiked the fall before his junior year at Seton Hall, and then his elbow blew out and he missed his entire junior season due to Tommy John surgery. This is the root of his sleeper status, but he'll likely bring his average four-pitch mix to Cincinnati at some point this year, and he has a chance to be a mainstay at the back end of the Reds' rotation.
Colorado Rockies: RHP Yency Almonte. Almonte's fastball lacks great plane, but it hums in at 93-96 mph and he'll flash an above-average changeup and good two-plane breaking ball. He merits continued development as a starter, but he has the stuff to succeed in a relief role right now if the Rockies need big league help.
Los Angeles Dodgers: RHP Wilmer Font. Font made his MLB debut in 2012 but is still prospect-eligible because he has thrown just seven total big league innings in the intervening years. He's back on the radar after leading the PCL in strikeouts last season. His fastball velocity sits at 92-95 mph to go with a plus slider and has a chance to make an impact in the Dodgers' bullpen.
Miami Marlins: OF Braxton Lee. Lee was acquired from Tampa Bay last year as part of the Adeiny Hechavarria trade. He's a plus-plus runner with zero power but has a discerning eye for the strike zone and is tough to strike out. If you've missed watching Juan Pierre play ball in Miami, you're going to enjoy Lee's game, and maybe soon, as Lee is one of just three outfielders currently on the Marlins' 40-man roster.
Milwaukee Brewers: RHP Adrian Houser. Houser made his big league debut in 2015, but poorly timed injuries have limited his reps since then. He spent much of 2017 rehabbing in Arizona while rooming with Diamondbacks righty Archie Bradley, and Houser looked incredible in the fall during instructional and Arizona Fall League. His fastball/curveball combination is enough to make him an effective reliever, but his changeup and cutter (though, I didn't see one last fall) have also been good in the past. If Milwaukee doesn't land starting pitching on the free-agent market, perhaps Houser will get a look every fifth day.
New York Mets: RHP Marcos Molina. The Mets' pitching staff hasn't exactly been a model of perfect health during the last several seasons, so there's a good chance Molina, despite health issues of his own, gets a chance to bring his deep arsenal of pitches to Queens this year. He has a plus fastball and slider, an average cutter and changeup, and a bad curveball.
Philadelphia Phillies: RHP Seranthony Dominguez. Dominguez has the stuff to start -- four pitches, including a plus slider and mid-90s fastball -- but injuries have limited him to about 60 innings each of the last two seasons. It makes sense for Philadelphia to continue developing Dominguez as a starter because the Phillies are rebuilding and don't need the immediate relief help he'd provide, but he could probably succeed in the MLB bullpen right now just on stuff if given the opportunity -- and he might get one in September.
Pittsburgh Pirates: RHP Jordan Milbrath. Milbrath lowered his arm slot last year, and it allowed his fringy slider to play up against right-handed hitters, to say nothing of his mid-90s fastball. He was Pittsburgh's Rule 5 selection and should stick as a righty specialist.
St. Louis Cardinals: SS Edmundo Sosa. There are scouts who look at the Cardinals' 40-man and see just one actual shortstop, Sosa. Sosa has average range and arm strength but elite defensive hands and actions. He probably won't do enough with the bat to be more than a career utility guy, but he's also the best defensive infielder the Cardinals have right now.
San Diego Padres: OF Franchy Cordero. There's real doubt about Franchy's baseball aptitude but not about his physical tools. He's a plus-plus runner with monster bat speed, and if he ever becomes even kind of selective, he could hit for big power. Things aren't likely to click for him, but if they do, Cordero could blow up.
San Francisco Giants: RHP Reyes Moronta. Moronta is basically a big league-ready relief asset right now. He sits at 93-97 mph with his fastball and has a plus slider.
Washington Nationals: INF Kelvin Gutierrez. Gutierrez has a minimalist swing that produces modest power and above-average rates of contact. Primarily a third baseman (though not an especially good one) Gutierrez got Arizona Fall League reps at first base and could be called upon if something happens to either Anthony Rendon or Ryan Zimmerman. His fringe glove and power limit his long-term ceiling, but Gutierrez is close to being a big league contributor of some kind.
American League
Baltimore Orioles: LHP Tanner Scott. Scott throws really hard for a lefty, touching 102 mph with the fastball and sitting 95-99. His changeup blew scouts away during the Fall League, and they consider it now well ahead of his slider. He might need to develop another half-grade of command to get there, but Scott has late-inning stuff.
Boston Red Sox: Utility man Tzu-Wei Lin. Lin has always been a capable defensive shortstop (maybe his arm is a little light for short) who couldn't make any impact contact, but an added leg kick in 2017 increased his ability to hit the ball in the air, and his all-around package of skills now suggests he could be a versatile utility player.
Chicago White Sox: SS Jose Rondon. Rondon will make some spectacular plays at shortstop and yank out the occasional home run thanks to his average raw power. He has never really developed an approach, and his overall offensive profile limits him to a utility infield role, but he might get a chance to take on that role in 2018 as the White Sox 40-man has several 1B/DH-only players. They'll need roster flexibility elsewhere.
Cleveland Indians: C Eric Haase. Haase hit .260/.352/.578 in the minors last year and scouts think he's OK behind the plate. With Francisco Mejia penciled in for a move to third base, it might just take an injury to Yan Gomes or Roberto Perez for Haase to be in the majors. Some scouts think he'll eventually be an everyday player.
Detroit Tigers: OF Mike Gerber. Gerber struggles against left-handed pitching, but he rakes against righties and plays an above-average defensive right field. He could make up the larger half of an outfield platoon with Mikie Mahtook by midseason.
Houston Astros: LHP Reymin Guduan. The Astros need lefty bullpen depth to come from somewhere, either Guduan, Anthony Gose or perhaps Framber Valdez. Gose will probably get every opportunity to win a job during the spring because he was the club's Rule 5 pick, but if Houston wants to carry another lefty, Guduan seems like the best fit. His fastball sits at 93-97 mph, and he mixes in an above-average slider.
Kansas City Royals: CF Bubba Starling. With Lorenzo Cain gone, Starling may finally get a chance to roam center field in Kauffman Stadium for the first time since he was drafted in 2011. He's a good defensive center fielder and has a plus arm but has never been able to hit.
Los Angeles Angels: RHP Osmer Morales. Morales was signed as a minor league free agent two offseasons ago and has put up good numbers in Triple-A. His strike-throwing ability and rainbow curveball should be enough for him to make competent spot starts for a thin Angels rotation this year, but his lack of velocity probably caps his ceiling at something like a No. 5 or 6 starter.
Minnesota Twins: LHP Lewis Thorpe. Thorpe hadn't pitched since 2014 entering last year, but his low-90s fastball and above-average changeup were back during a healthy 2017. He also throws a curve and slider, but his medical history and fringy command point to a bullpen role.
New York Yankees: 1B Billy McKinney. McKinney began transitioning from the outfield to first base last season. Like so many others, he has altered his swing in such a way that it has unlocked dormant in-game power. With Mike Ford and Garrett Cooper both gone, the competition for reps at first base should Greg Bird go down largely consists of McKinney.
Oakland Athletics: Ramon Laureano, OF. Laureano had a breakout 2016 and then stumbled early last year before rebounding over his final 200 plate appearances. Houston had a significant upper-level outfield crunch from which Derek Fisher and Jake Marisnick have emerged, leaving Laureano, Jason Martin and Teoscar Hernandez as trade fodder. Scouts thought Laureano had a shot to be a do-it-all everyday player after his 2016 campaign, so he was an interesting buy-low option for Oakland.
Seattle Mariners: 1B Mike Ford. Ford might end up being the 2018 season's most interesting rookie. He was Ivy League Pitcher and Player of the Year at Princeton but went undrafted. He's not especially toolsy and doesn't have huge raw power, but Ford tracks pitches well, has great feel for the strike zone and grinds out tough at-bats. He might reach base enough to be a 1B/DH contributor even without big power.
Tampa Bay Rays: LHP Jose Alvarado. He doesn't really know where it's going, but Alvarado has a plus-plus fastball and curveball combination that will play at the back of a bullpen if he ever learns to harness it. After making his debut last season, he should spend much of 2018 in the big leagues.
Texas Rangers: LHP Brett Martin. Injuries have plagued Martin for three straight seasons, but he shows a good four-pitch mix when healthy, led by an old-school overhand curveball. If Texas needs lefty bullpen help, the Rangers could look to Martin instead of lefties higher on the depth chart, like Yohander Mendez, who they could prefer to continue developing as a starter.
Toronto Blue Jays: C Danny Jansen. Apparently, seeing the baseball is an important aspect of hitting. Jansen drew disparate reviews from scouts when he was in the low minors, but he started wearing glasses after the 2016 season and had a breakout 2017, during which he cut his strikeout rate in half. He had a strong cup of coffee at Triple-A late in 2017 and could carry his newfound success to the majors next year.