On zero or one day's rest this season, Soria's combined ERA is 4.45. On two or three days' rest, the combined ERA is 0.98.
On two days' rest Saturday, Soria pitched a 1-2-3 inning against the White Sox.
"He had a great, 11-pitch inning," Yost said. "Right now I'd say he could go on one day rest if we need him. But if he has a 23- or 24-pitch inning, we might look to go two days' rest with him. Right now he's raring and ready to go."
But Soria points out that the rest issue hasn't been a factor in the past. His career ERA on zero rest is 2.66.
"I just think it's a different season," Soria said. "Every season there's something else you can look up in terms of stats.
"But I'll do whatever [Yost] wants me to do. Maybe [Yost] has found something that will help us win. That's all that matters."
Royals call up Mills
Prior to Monday's 16-3 defeat to Oakland, the Royals called up their No. 9 prospect, right-hander Alec Mills from Triple-A Omaha.
Mills entered the game in the ninth inning and gave up four runs.
Mills made his Major League debut for the Royals on May 18, pitching two-thirds of an inning in a 5-2 loss to Boston.
In 24 games (23 starts) combined between Northwest Arkansas (Double-A) and Omaha this season, he went 5-5 with a 3.22 ERA with 122 strikeouts in 125 2/3 innings.
"He was going to come up last week, but he got sick," Yost said.
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.