Fogg, sidelined since June 1 with back problems, was activated in time to start for Cincinnati on Saturday night in place of right-hander Aaron Harang against Washington. Harang's next start was pushed back to Tuesday against the Cubs in Chicago because of stiffness in a forearm muscle.
Fogg was 1-2 with a 9.85 ERA in 11 games, including four starts, before Saturday's outing.
Hopper, limited to 26 games due to elbow problems, will undergo ulnar collateral ligament replacement surgery on Tuesday, Cincinnati manager Dusty Baker said.
Hopper was also on the disabled list from April 23 to June 18.