Rusin, 26, is 7-7 with a 3.27 ERA (40 ER/110.0 IP) in 17 appearances (16 starts) with Iowa this season. He has limited opponents to a .241 batting average (99-for-410), including a .242 average against lefties and a .241 mark against righties. Rusin has walked only 22 batters in 110.0 innings, good for an average of just 1.8 walks per nine innings.
The 6-foot-2, 195-pound Rusin made his big league debut with the Cubs last season, going 2-3 with a 6.37 ERA (21 ER/29.2 IP) in seven starts. He allowed two earned runs or less in four of his outings. Rusin spent the majority of the 2012 season with Iowa, his first full season at Triple-A, and went 8-9 with a 4.55 ERA (71 ER/140.1 IP) in 25 starts. He was originally selected by the Cubs in the fourth round of the 2009 Draft out of the University of Kentucky.
Sappelt, 26, was a member of Chicago's 2013 Opening Day roster and batted .178 (8-for-45) in 20 games before being optioned to Iowa on May 6. He is batting .269 (50-for-186) with 12 doubles, three triples, two home runs and 34 RBI in 52 games with Iowa this season.
The right-handed hitting outfielder joined the Cubs prior to the 2012 season with lefthander Travis Wood and minor league infielder Ronald Torreyes in the trade that sent Sean Marshall to Cincinnati. Sappelt batted .275 (19-for-69) in 26 games for the Cubs last season.
Sweeney, 28, endured a left rib fracture when crashing into the center field wall during Saturday's game in Seattle. He is batting .295 (33-for-112) with four home runs and 15 RBI in 44 games with the Cubs this season.
Chicago's 40-man roster stands at 40 players.