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.