The Reds activated Williamson (shoulder) from the 60-day injured list Tuesday.
Williamson is slated to make a spot start Wednesday versus the Padres, and will be used in a piggyback role with Chase Burns afterwards. The left-hander held a 6.11 ERA and 19:20 K:BB over 28 frames in six starts for the Reds earlier this season and has missed a chunk of the year with a shoulder issue.