Fields, 28, a one-time top pick of the White Sox, hit .306 with three home runs and six RBIs in 13 games with the Royals last year. This spring, he was a non-roster invitee of the Pirates who already had been reassigned to Minor League camp.
Fields, who played with the White Sox 2006-09, has appeared in 217 Major League games, mostly at third base (158 games). He has a .234 career batting average with 34 home runs and 107 RBIs in 713 at-bats.
The Rockies have Ian Stewart at third at the Major League level, with Ty Wigginton, Jonathan Herrera and Jose Lopez all possessing the ability to move there in a pinch. Fields could be a key part of the lineup at Triple-A Colorado Springs.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.