 Jeffress was called up to the majors for the first time on September 1, 2010.He made his major league debut that evening, pitching one scoreless inning.On December 18, 2010 Jeffress was traded to the Kansas City Royals with Alcides Escobar, Lorenzo Cain, and Jake Odorizzi for Zack Greinke and Yuniesky Betancourt.