Robertson, 25, had a 3.05 ERA with 20 strikeouts and 16 walks in 20 2/3 innings at Triple-A Rochester this year. The 6-foot-5 lefty started the year with the Twins but was optioned after just two appearances. He allowed one earned run in one inning of work but also allowed six inherited runners to score, including three on a grand slam from Baltimore's Chris Davis.
He has a 5.54 ERA in 26 career innings in the Majors and a 3.68 ERA in 665 2/3 innings in the Minor Leagues. He was originally selected by the club in the third round of the 2006 First-Year Player Draft and began his career as a starter until being converted to a reliever in '11.
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.