Keller was a top prospect for the Arizona Diamondbacks, ranked by MLB.com as No. 12 in the whole organization, but he was left off the Diamondbacks 40 man roster last month.
Keller made 26 stars in Double-A, throwing 130 2/3 innings with a 4.68 ERA, and averaging 7.7 strikeouts per nine innings and 3.9 walks. Also involved in the trade were the Mets, who sent Burch Smith to the Royals, who was sidelined for two years following Tommy John surgery. Both the Mets and Reds will receive a player to be named later.