The move with Cahill was made retroactive to Sunday. Cahill is 1-1 this season with a 2.25 ERA in four starts.
He suffered an open rupture of the patellar tendon in his right knee last year in his major league debut for the Yankees when he crashed into the first-base line rail at Guaranteed Rate Field in Chicago.
Fowler appeared as a pinch-hitter Wednesday in the seventh inning of the A's 4-1 loss to the Houston Astros and lined out to second base. He remained in the game and played center field.