Buckner, 27, began last season at Triple-A Reno (3-1, 3.53 ERA) before going 0-3 with an 11.08 ERA in three starts with the D-backs last season. The D-backs sent him to the Tigers for pitcher Dontrelle Willis on June 1. The Tigers released him from Triple-A Toledo on July 25.
Buckner is 6-11 with a 6.25 ERA in 36 Major League games, including 21 starts.
"We've seen a lot of him, and he's a hard worker and he still has some upside as a prospect," Rockies player development director Marc Gustafson said.
Buckner joins first baseman Mike Jacobs, catcher Matt Pagnozzi and left-handed pitcher Eric Stults as veterans who are receiving non-roster invitations. The Rockies also have signed former Yankees top pick Eric Duncan, an infielder, and former Giants pitcher Geno Espineli to straight Minor League deals.
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.