In December 2010, the Royals waived Humber, and he was claimed by the Oakland Athletics, who had extra room on their 40-man roster at the time of the waiver claim. However, the team subsequently filled its 40-man roster and designated Humber for assignment later that off-season in order to make room on their roster for Guillermo Moscoso, whom the Athletics acquired from the Texas Rangers in a trade during January 2011. The Chicago White Sox claimed Humber off waivers from the Athletics on January 18, 2011. He signed a contract worth $500,000 for the season. White Sox pitching coach Don Cooper worked with Humber, as he replaced his cut fastball with a slider and improved his pitching mechanics in the fashion originally recommended by Peterson in 2005.