Stewart would likely pitch on Friday, if he did start again, but he could be needed out of the bullpen before then, which would be the most likely thing keeping Stewart from making a second start.
"I'm thinking, personally, we should keep him and give those guys six days," Guillen said. "I don't know yet, but ... I have to know in the next couple days because I have to know exactly who I'm going to have in the bullpen and how I'm going to use them."
Stewart allowed just one run on eight hits over 6 1/3 innings Saturday night in in a 6-1 victory to pick up his first Major League win.
While it remains a possibility that Stewart could start against the Royals, the right-hander would not start again the next time through the rotation, because the White Sox have off-days on Aug. 15 and Aug. 22, and using a six-man rotation through that stretch would give everyone too much time between starts.
Guillen said he would talk to White Sox general manager Ken Williams before making a decision on when Stewart would pitch next.