The Cubs had acquired Volstad from the Marlins for Carlos Zambrano in January, and the right-hander opened the season in the rotation but went 0-7 with a 7.94 ERA in nine starts before he was optioned to Iowa. Volstad rejoined the Cubs Aug. 1, finished 3-5 with a 5.18 ERA in his final 12 starts, and was 3-12 with a 6.31 ERA for the season.
He ended a 24-start winless streak on Aug. 26 with a win over the Rockies, his first win since July 10, 2011.
The Cubs front office had made it clear they were in the market for two starting pitchers and that Volstad was not projected as part of the 2013 rotation.
Besides Mather, right-handers Manuel Corpas, Justin Germano and Blake Parker were outrighted to Triple-A Iowa. Mather, who started in all three outfield spots and third base, batted .209 in 103 games with the Cubs.
Corpas, who was coming back from Tommy John surgery, which he had in September 2010, was 0-2 with a 5.01 ERA in 48 games. Corpas has elected free agency.
Germano, acquired from the Red Sox July 19 for cash considerations, went 2-10 with a 6.75 ERA in 13 games (12 starts). Parker spent most of the season on the disabled list with an elbow injury, appearing in just seven games.
On Thursday, the Cubs outrighted infielder Adrian Cardenas and right-handed pitchers Miguel Socolovich, Jason Berken and Marcos Mateo were assigned outright to Iowa. Catcher Anthony Recker also was claimed off waivers by the Mets.
The Cubs' 40-man roster is now at 33.
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.