Grimm began the season with the Cubs but was optioned on Friday because the club needed a fresh arm in the bullpen heading into the Yankees series.
La Stella began the season with the Cubs before being optioned on April 22. With Iowa, La Stella hit .267 with a double, a home run and two RBIs in 12 games. He found out about the callup after homering in Saturday's game.
"It's a little different approach, being down there [in the Minor Leagues] and getting four or five [at-bats] every night," La Stella said. "You take a little different approach than when you get the one [at-bat] in the seventh and you don't want to miss it. You want to establish yourself and get in the game flow."
Zastryzny was promoted on Saturday from Iowa, and gave up three runs in a career-high four innings. He has a 6.88 ERA in seven relief appearances with Iowa this season.
Carrie Muskat has covered the Cubs since 1987, and for MLB.com since 2001. She writes a blog, Muskat Ramblings. You can follow her on Twitter @CarrieMuskat and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.