Castellanos, 27, went on a tear after being acquired by the Chicago Cubs from the Detroit Tigers prior to the trade deadline last year. With the Cubs, the outfielder hit .321/.356/.646 with 16 homers and 21 doubles over 51 games.
By virtue of being traded midseason, Castellanos was not eligible to receive a qualifying offer - unlike Ozuna - and is therefore not attached to draft-pick compensation. Ozuna wound up reportedly landing a one-year, $18-million contract with the Braves.
Earlier in the offseason, Castellanos was said to be pursuing a winning culture and preferred to sign with the Rangers, Cubs, or San Francisco Giants.
The Reds have already had a busy offseason, adding Mike Moustakas and Shogo Akiyama to their lineup. Akiyama and Aristides Aquino are expected to be everyday outfielders for the club, while Nick Senzel could feature prominently as well once he is fully healthy following offseason shoulder surgery.
Meanwhile, the Rangers have a bit of a logjam of their own in the outfield, with Joey Gallo, Danny Santana, and Willie Calhoun all vying to be everyday hitters. However, Gallo could return to first base to accommodate the addition of Castellanos.