Rusin, 27, has spent the majority of the season with Iowa, going 7-11 with a 3.76 ERA in 19 starts, including a no-hitter on May 7 in New Orleans.
In two relief appearances with the Cubs this season, Rusin has given up six earned runs over 8 2/3 innings.
Parker, 29, has appeared in seven games, spanning four stints with the Cubs this season. He has spent the majority of the year with Iowa, posting a 1.29 ERA with a Pacific Coast League-leading 21 saves in 28 games.
Parker was one of eight pitchers, including backup catcher John Baker, to pitch on Tuesday in the Cubs' 16-inning game, a 4-3 win over the Rockies.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.