The 24-year-old Jeffress appeared in two games for the Royals in late April. He pitched a total of one inning and allowed four hits but no runs. Last season, Jeffress pitched in 14 games with the Royals, posting an ERA of 4.70.
In 37 games with Omaha this season, Jeffress is 5-4 with a 4.97 ERA. He has struck out 61 batters in 58 innings.
Manager Ned Yost said Jeffress would slide into the bullpen and pitch in a middle-relief role. He said the decision to recall Jeffress stemmed from increased consistency from the righty.
"Command has been an issue, but consistency in command has been an issue. He's been a lot more consistent," Yost said. "He'll still have his hiccup here and there, but for the most part, he's been very consistent in one- or two-inning stints, coming in throwing strikes with his fastball and with his breaking ball. We want to give him a look."
Vinnie Duber is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.