In an unrelated roster move, infielder Blake DeWitt has been assigned outright to the Triple-A Iowa roster.
Maine, 27, joins the Cubs for his second stint this season after going 0-1 with a 1.59 ERA (1 ER/5.2 IP) in five relief appearances with the big league club from April 20-May 3. He has struck out 10 batters in 5.2 innings pitched and limited opponents to a .056 batting average, including a 1-for-15 (.067) effort by right-handed hitters and a 0-for-3 effort by lefthanders.
With Iowa this season, Maine is 2-1 with two saves and a 5.25 ERA (7 ER/12.0 IP) in 10 relief appearances, holding foes to a .163 batting average.
Maine is 0-1 with a 4.21 ERA (12 ER/25.2 IP) in 25 major league relief appearances covering the past three seasons with the Cubs (2010-12). He made his big league debut in 2010, posting a 2.08 ERA (3 ER/13.0 IP) in 13 outings before recording a 10.29 ERA (8 ER/7.0 IP) in seven appearances last year. Maine spent most of the 2011 campaign with Triple-A Iowa, where he went 3-4 with 12 saves and a 3.68 ERA (21 ER/51.1 IP) in 38 relief outings.
Castillo, 23, is 0-1 with a 7.04 ERA (6 ER/7.2 IP) in seven relief appearances with the Cubs this season.