Moustakas, who has been battling tightness in his left calf, also missed Tuesday's contest against the Marlins. The Royals claim that the walking boot is a precautionary measure to relieve pressure on his calf.
The 24-year-old Moustakas got off to a rough start this season but has improved of late, batting .293 with four home runs and 13 RBIs since the All-Star break.
The Royals did not say whether Moustakas would return Thursday for the opener of a crucial five-game series against the Detroit Tigers. Kansas City (62-55) entered play Wednesday in third place in the AL Central, just 6½ games behind first-place Detroit (69-49).
Royals manager Ned Yost was asked whether Moustakas would land on the disabled list after hurting his calf rounding third base earlier in the week.
"It's possible. Everything is possible, but not probable," Yost said. "He's feeling better. We're going to reevaluate probably tomorrow and see where we're at, but I don't anticipate him having to go on the DL."
Elliot Johnson started at third base for Wednesday's matinee contest against the Marlins. Fellow infielder Miguel Tejada, who had been playing second and third quite regularly, went on the disabled list Sunday with a strained right calf.
The Royals have streaked into playoff contention for the first time in a decade, ripping off 17 wins in 20 games before a 1-0 loss in extra innings to the Marlins on Tuesday night.
Along with losing Tejada and the injury to Moustakas, the Royals are also playing without center fielder Lorenzo Cain, who went on the disabled list after straining his left oblique last Friday.
"I've been through this, many, many times. It's all part of the game," Yost said of the sudden rash of injuries. "You have to be able to deal with it. You can't sit back and complain about it, worry about it -- `Woe is me. Why is this happening?'
"You deal with it and move on," he added. "We feel like we've got enough depth to cover it. None of these injuries are long-term injuries, knock on wood, but just little nicks and dings."
Information from The Associated Press was used in this report.