- Doug Padilla, Chicago White Sox beat reporter
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Castillo has been out since May 11 with a left groin strain and just completed a lengthy minor-league rehab assignment.
In seven outings earlier this season, the right-handed Castillo had a 7.40 ERA. Maine is 1-1 with a 4.79 ERA over three separate stints with the Cubs this season.
Castillo was a Rule 5 selection from the Phillies this past winter and must be on the major-league roster all season or be offered back to his former club.
Out since early May, Castillo finally activated while Maine is optioned to Iowa.