To create room on the active roster, the Giants optioned reliever George Kontos to Triple-A Fresno. Kontos was 1-0 with a 3.07 ERA in 11 appearances with the Giants.
"Not sure we'll need the extra pitcher with the day off plus we'll be going with the [designated hitter] so you don't use as many pitchers when you use the DH," Bochy said before Sunday's game.
Casilla's return gives Bochy a good problem to have, creating a bit of a logjam in the bullpen, with non-closer right-handers Casilla, Juan Gutierrez and Jean Machi all having strong seasons in addition to lefty Javier Lopez.
Casilla will return to his role as setup man, bouncing Machi -- owner of a ridiculous 0.29 ERA through 31 innings pitched this season -- back to more of a sixth- and seventh-inning role.
The Giants have off-days Monday and Thursday, sandwiching a two-game series against the American League's White Sox on Tuesday and Wednesday.
Casilla owns a 1.37 ERA through 26 1/3 innings. He strained his right hamstring on May 21.
Ryan Hood is an associate reporter for MLB.com. Follow him on Twitter @ryanhood19. This story was not subject to the approval of Major League Baseball or its clubs.