Rated the No. 10 free agent entering the offseason by MLB.com, Castellanos is a right-handed hitting outfielder who hit .289 with 27 homers and 73 RBIs last season playing for the Tigers and Cubs.
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Drafted out of high school 44th overall in 2010 by the Tigers, Castellanos will be 28 on Opening Day.
He made his MLB debut in 2013 and has a lifetime batting average of .277 with 120 home runs, 229 doubles, 32 triples and 460 RBIs in 888 games.
His lifetime OPS is .797, and he is coming off a career year in which he posted an .863 OPS and hit a major-league best 58 doubles.
The Cubs acquired Castellanos at the deadline last summer for a pair of minor leaguers.
He has played over 500 games at third base but only appeared in the outfield the past two seasons — primarily right field.
The Reds have had a busy offseason trying to build on a 2019 season in which they improved by eight wins to 75-87 and snapped a streak of five straight seasons in the NL Central basement.
They previously signed second baseman Mike Moustakas to a four-year, $64 million contract, outfielder Shogo Akiyama to a three-year, $21 million deal and left-handed pitcher Wade Miley to a two-year $15 million deal.
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Miley joins a rotation that already was viewed as a strength with Luis Castillo, Sonny Gray, Trevor Bauer and Anthony DeSclafani while Moustakas is expected to hold down second base or the foreseeable future.
Akiyama and Castellanos join an outfield that was already crowded with candidates but short on long-time established players.
Top prospect Nick Senzel is expected to remain in center field — unless he is traded — but who gets the most at-bats from a group that also includes Jesse Winker, Aristides Aquino, Phil Ervin, Scott Schebler and Travis Jankowski remains to be seen.
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Winker has a reputation as an advanced hitter, but he has had a hard time staying healthy — as is the case with Senzel throughout his time in the Reds organization.
After news of the Castellanos signing, Heyman tweeted the Reds are “considering” trading Senzel, who made his MLB debut last season after three in the minors.
Earlier in the winter, the Reds were reported to be interested in acquiring Francisco Lindor from the Indians or fellow shortstop Corey Seager from the Dodgers, deals that would in all likelihood involve Senzel and other major prospects in the Reds system.
Aquino hit 19 home runs in 56 games last season, following a historic August with a rough September.