Peavy has been out since May 27 with a strained right forearm. The 23-year-old right-hander last pitched on May 19, when he held Pittsburgh to one run over six innings while striking out a season-high nine. At the time of his injury, Peavy was leading the National League with a 2.01 ERA.
“I’m going out there tomorrow night and obviously I’m not going to be as sharp as I was after the 10 or so starts I had before I got hurt,” Peavy said. “I’ll have to just stay cool, calm and collected and make pitches.”
The Padres are coming off a four-game split with Arizona, capped by Thursday’s 7-5 loss.
Coming off back-to-back last-place finishes, the Padres have won seven of their last 10 overall, and are just 2 1/2 games behind first-place San Francisco in the NL West.
“I like what we’ve got going on, what moves we make if we make any,” Peavy added.
The Royals, who finished 83-79 last season for their first winning season since 1993, are in last place in the AL Central.
Kansas City has lost six of seven overall, including a 3-2 loss to Baltimore on Thursday.