Mock, 25, went 2-1 with a 2.52 ERA in four starts with Columbus in July, improving his season record to 6-3 with a 3.05 ERA in 17 starts. This marks his third stint with the Nationals this season, following two spot starts in June.
The Nationals are planning to use Mock in the bullpen for the time being. Manager Manny Acta said the organization still considers Mock a starter.
Bonifacio was acquired Tuesday from Arizona in exchange for closer Jon Rauch.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.