Fantasy baseball's 2018 All-Sleeper team

Fantasy owners know there will always be players at each position who can make or break a season by performing much better -- or much worse -- than expected. Here are our favorite sleeper picks for the 2018 season.
Catcher
Mike Zunino, Seattle Mariners: Not a top-tier option, but his .509 slugging percentage ranked second among catchers last season
First base
Matt Olson, Oakland A’s: Only played in 59 games, but no one hit homers at a higher rate than he did (one every nine plate appearances).
Second base
Yoan Moncada, Chicago White Sox: Classic post-hype prospect. Hit .276 with five homers over season’s final month and has 30-steal speed if allowed to run.
Shortstop
Jorge Polanco, Minnesota Twins: Very quietly hit .293/.359/.511 with 10 homers in the second half … at age 23.
Third base
Nicholas Castellanos, Detroit Tigers: Improved his slugging percentage and cut his strikeout rate in the second half; move to the outfield adds to his fantasy value.
Outfield
Gregory Polanco, Pittsburgh Pirates: On verge of a breakout in 2016, injuries set him back last season; significant drop in strikeout rate a good omen.
Starting pitcher
Luis Castillo, Cincinnati Reds: Scouts love his upper 90s velocity and his three plus pitches; shouldn’t have any workload restrictions in his second MLB season.
Relief pitcher
Blake Treinen, Oakland A’s: Change of scenery worked wonders as he posted a 2.13 ERA, 9.9 K/9 and 13 saves after trade from Washington.
More fantasy:
Blessing: Top 15 rookies for 2018
Michaels: Most overvalued and undervalued players
Gardner: This season's burning fantasy questions