Davies will start on Saturday night against the Cleveland Indians at Kauffman Stadium. Tomko was reassigned to the bullpen, effective immediately. Both are right-handers.
Tomko, in 11 games and 10 starts for the Royals, has a 2-6 record and a 6.11 ERA. Davies will be recalled from Triple-A Omaha, where he was 6-2 with a 2.06 ERA in 10 starts.
"We appreciate everything that Brett's done but, in light of what we're going through right now, obviously the numbers are what they are," manager Trey Hillman said Thursday.
"He's had an inability to stay away from a big inning. There are some statistics that we've studied that lead us to believe he'll have more of an impact in our bullpen. We'll utilize him in the seventh and eighth inning, and try to help out for the Leo [Nunez] situation."
Nunez went on the disabled list and left-hander Neal Musser was recalled from Triple-A Omaha after Wednesday night's loss -- the Royals' 10th in a row.
Davies, 24, last pitched on Monday night, when he held Round Rock to one run in seven innings. He's allowed just four earned runs in his last 25 innings. Over his total 56 2/3 innings, he's struck out 37 and walked 20.
Last season, Davies was acquired on July 31 from the Atlanta Braves for reliever Octavio Dotel. He made 11 starts for the Royals and went 3-7 with a 6.66 ERA.
The biggest drawback in his career has been an inability to exercise good command with his fastball.
In parts of four seasons with the Braves and Royals, his record was 17-28 with a 6.24 ERA. He's made 56 Major League starts.
When Davies goes on the active list on Saturday, a pitcher will have to go, and relievers Yasuhiko Yabuta and Joel Peralta could be at risk.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.