De Jong was acquired last summer, then designated and outrighted over the winter. He got another shot in the majors earlier this year, but was torched in his lone outing before again being dropped from the 40-man roster.
The season has been a struggle thus far for the 25-year-old, who was a 2012 second-round pick. He has allowed more than an earned run per inning at Triple-A. Through 45 1/3 innings at the highest level of the minors, he has allowed nearly as many walks (26) as he has recorded strikeouts (30) while surrendering 16 long balls.