The A's also announced Monday that they have claimed outfielder Kenny Wilson off waivers from Toronto and optioned him to Triple-A Sacramento. In addition, Oakland outrighted outfielder Nick Buss to Sacramento and traded International Slot No. 57, worth $339,000, to Milwaukee for minor league pitcher Rodolfo Fernandez.
Mills went 1-1 with a 4.41 ERA in three starts for Oakland this season. But with Hammel scheduled to pitch Wednesday at San Francisco, the A's needed to clear space on the roster.
The A's got Hammel and right-hander Jeff Samardzija from the Cubs on Friday in a trade that sent three prospects to Chicago.