Holland, who was on the 15-day DL with a rib injury, struck out three and walked one in the eighth inning of Kansas City's 5-0 win Saturday against the White Sox.
"He threw the ball good," manager Ned Yost said. "He looked like Greg Holland of last year and that's always encouraging."
Holland slides back into his usual late-inning role, giving the Royals more flexibility in the seventh, eighth and ninth innings. Adcock was a long reliever.
"A lot," Royals manager Ned Yost said of how much Holland's return helps the bullpen. "You need other quality arms down there. ... We've eliminated one of the long guys and now have three quality righties to go with two quality lefties. That helps a lot."
Following Saturday, Holland is 0-2 with a 9.02 ERA in eight appearances this season. He completed two rehab appearances before returning, the most recent on Thursday for Double-A Northwest Arkansas when he struck out all three batters he faced in the first inning.
Adcock, who was recalled from Omaha on April 28, posted a 2.89 ERA in three appearances for the Royals. The 24-year-old was 3-1 with a 1.37 ERA in four Triple-A starts, and Yost said the organization opted to send down Adcock so he could continue his development.
"He's just young. He's doing fine and we don't want him sitting. We want him pitching," Yost said. "He's making great improvements and making great strides. We need him down in Triple-A throwing the ball every five days in case we have a need option. He's a viable option for a callup if we need it."