Castillo, a Rule 5 selection who can't be sent down without first being offered back to his former team, the Philadelphia Phillies, has a 7.04 ERA in limited use this season. He has made just seven appearances, his last Friday at Milwaukee when he hit two batters and took the loss in a 13-inning game.
Maine pitched well in an earlier stint with the Cubs. He posted a 1.59 ERA in five appearances but got caught in a roster numbers crunch and had to be sent back down May 3.
Maine got into Wednesday's game against the Philadelphia Phillies, and did not record an out in the ninth inning. He surrendered a double, hit a batter and walked another before being relieved by Michael Bowden.