John Lackey's previously announced return from the disabled list became official just a couple hours before his start against the Atlanta Braves.
In order to free up the roster spot for the veteran righty, the Cubs sent Eddie Butler to Triple-A Iowa.
Butler made 11 starts and 13 appearances filling in while Lackey and Kyle Hendricks spent time on the DL. Butler is 4-3 with a 3.95 ERA this season.
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Butler was already boxed out of the Cubs' starting rotation when the team acquired Jose Quintana in last week's blockbuster trade, making his second relief appearance of the season in Saturday's win over the Baltimore Orioles.
Butler made it out of the sixth inning in just one of his 11 starts, his first back on May 12. Longevity was a persistent problem, as he failed to make it out of the fifth inning five times.
Lackey returns Tuesday night for his first start since July 5. He's 5-9 on the season with a 5.20 ERA. Lackey has allowed the most home runs of any National League pitcher this season with 24.