De Leon was optioned to Triple-A Louisville on Sunday.
De Leon was acquired from the Rays in November and had an outside chance at cracking the Opening Day roster, but he's now set to begin the season -- whenever that occurs -- in the minors. The 27-year-old had a 3.51 ERA, 1.33 WHIP and 73:27 K:BB over 51.1 innings at Triple-A last season, though he also had three big-league appearances.