Jeffress pitched two-thirds of an inning in Sunday's 8-6 loss and gave up three hits, a walk and two unearned runs while throwing 28 pitches. In eight games in the Majors this season, Jeffress has yet to be charged with an earned run while pitching 6 2/3 innings.
Jeffress was sent to Double-A because Northwest Arkansas' schedule ends on Sept. 3 and he could return to the Royals then instead of waiting the usual 10 days before being recalled.
Yost ruled out using relievers Aaron Crow and Tim Collins on Monday because they'd both pitched in all three previous games vs. the Red Sox.
Mazzaro was rested, having not pitched for Omaha since last Tuesday. For the Storm Chasers, he had a 2-2 record, five saves and a 3.63 ERA in 22 games, including eight starts.
"He's been throwing fine in relief, throwing strikes. He's been pitching the back end," Yost said.
This is the fourth time that Mazzaro has been recalled from Omaha this season. In his previous three stays with the Royals, he was 3-3 with a 6.12 ERA in nine games, six of them starts.
Before facing the Red Sox, Royals starters had pitched six or more innings in 10 of 14 games. Two of the four short starts came against the Rays in two of the first three games of the club's road trip, but there was a day off before the Boston series.
"[Bruce] Chen went six, which was good, but then [Jeremy] Guthrie 2 2/3 and then five innings [by Will Smith] yesterday," Yost said, noting the Guthrie game went 12 innings. "That's why it was important for Will to get us through six yesterday; that would've made a big difference, but he couldn't do it."
So the call went out for Mazzaro.