Mengden moved into a bullpen role July 31 after the Athletics added Jesus Luzardo to the rotation, Susan Slusser of the San Francisco Chronicle reports.
Mengden hasn't been used since his last start July 28, while Oakland's top pitching prospect strengthened his hold on a rotation spot by tossing five shutout innings Tuesday against the Rangers. The Athletics presumably still view Mengden as the next man up in the rotation, but he'll be largely limited to low-leverage mop-up work while he's in the bullpen.