The Cubs also added right-handed pitcher Alberto Cabrera, 23, from Iowa, for a fresh arm in the bullpen and optioned Casey Coleman to the Triple-A team.
The Cubs have been juggling the rotation following the trades of starters Ryan Dempster and Paul Maholm. Jeff Samardzija will start on Friday against the Dodgers and was to be followed by Justin Germano. Matt Garza, who has not pitched since July 21, was not expected to throw another bullpen until this weekend, and most likely will not start until the Cubs face the Padres next week. Garza could celebrate as his wife gave birth to the couple's fourth child, a daughter, on Tuesday.
Volstad could go on Sunday. He was 3-5 with a 5.17 ERA in 12 starts with Iowa, and 0-7 with a 7.94 ERA with the Cubs this season.
In his last start July 27, Volstad threw eight shutout innings, giving up four hits and three walks and striking out five.
"I hope we can get out of him what he threw the other day," manager Dale Sveum said before Wednesday's series finale against the Pirates. "He sounded really confident about that start, and how he pitched the left-handers inside and went after everybody, and didn't think too much about anything and just dominated the game."
Volstad, acquired in the offseason from the Marlins for Carlos Zambrano, said he got a lot of confidence from his last game.
"It was just a matter of attacking, making them swing at my pitch the entire game and keeping going," he said. "It was good. I want to continue off that for sure."