The Cubs have played three extra-inning games in the first 10 and wanted depth, especially Saturday with Carlos Villanueva starting. Villanueva lasted three innings Saturday, and Rusin finished up, giving up one run on three hits over five innings in a 10-4 St. Louis win.
"We need some length," manager Rick Renteria said. "We've used the pen a lot."
Rusin, 27, was 0-2 with a 6.52 ERA in two starts with Iowa, and had been scheduled to start Sunday against Memphis. Instead, he was to rejoin the team as he was optioned back to Iowa. The Cubs were expected to recall right-handed reliever Blake Parker on Sunday.