Royals manager Ned Yost said a corresponding move to make room for Lopez on the 25-man roster would be made before Wednesday's game against the Blue Jays.
Lopez posted a 5.26 ERA in 26 outings at Triple-A this year between the Royals' and Brewers' organizations. In Lopez's last outing with Omaha, he threw 5 2/3 scoreless innings while giving up three hits and striking out seven against Salt Lake City.
"We got six weeks left [to the season]," Yost said. "He made a start the other day that was as good as he's had down there. We've got six weeks left and we want to look at him."
Right-hander Burch Smith, who had been in the rotation and was scheduled to start on Wednesday, will move back to the bullpen.
"I told Burch over the weekend that he was going back to the bullpen," Yost said. "He's been preparing for that. Tonight was his first night in the 'pen."