Gallagher (Watch Video) will wear uniform No. 36 and be available for this afternoon's game in St. Louis.
The 22-year-old Gallagher is 2-2 with a 3.10 ERA (10 ER/29.0 IP) in five starts with Iowa this season. He has limited opponents to a .196 batting average, including a .162 average to lefthanders and a .214 mark to righthanders, and has struck out 30 and walked only nine batters in 29.0 innings pitched.
Gallagher is the reigning Pacific Coast League Pitcher of the Week after allowing one run on three hits and one walk with 12 strikeouts in 7.2 innings in a 5-1 win on April 22 vs. Oklahoma. He most recently pitched Monday at Fresno, and allowed five runs in 5.0 innings and endured an 11-3 setback.
The righthander made his major league debut with the Cubs last season and posted no record, one save and an 8.59 ERA (14 ER/14.2 IP) in three stints in the big leagues. In 89 career minor league appearances, all but one as a starter, Gallagher is 38-17 with a 2.77 ERA (148 ER/481.0 IP). He has 482 strikeouts in 481.0 innings pitched.
Hill is 1-0 with a 4.12 ERA (9 ER/19.2 IP) in five starts with the Cubs this season, walking 18 batters while fanning 15 in 19.2 innings. He returns to Triple-A for the first time since the 2006 campaign.