Friedl's move is retroactive to Wednesday for the Reds, who transferred left-hander Nick Lodolo to the 60-day injured list.
Friedl, 27, is hitting .326 with three homers and 20 RBIs in 42 games with the Reds this season.
Hopkins, 26, is batting .341 with seven homers, 27 RBIs and 31 runs in 50 games this season with Louisville.
Lodolo, 25, is dealing with a stress reaction in his left tibia. He is 2-1 with a 6.29 ERA in seven starts this season.