The right-handed Jefferies, ranked by MLBPipeline.com as the organization's No. 8 prospect, was a second-round Draft pick of the A's just last year. The Cal Berkeley product made two appearances for Class A Stockton this season, tossing four scoreless innings in his debut before allowing six runs (two earned) in his next outing.
Jefferies, 21, was limited to 50 innings during his final season at Cal because of injuries, none involving his elbow.
Jane Lee has covered the A's for MLB.com since 2010.