Sierra has been promoted to Double-A Tulsa, J.P. Hoornstra of the Southern California News Group reports.
Sierra, who was converted to a reliever in July, has pitched well in seven relief appearances at High-A Rancho Cucamonga, giving up only four runs in 14 innings. Double-A will be tough test for the 25-year-old, who has never pitched above High-A in the minors.