Mazzaro, a starter with Triple-A Omaha, was brought up to work in the bullpen. Left-handed reliever Tommy Hottovy was sent back to Omaha to make room for Mazzaro, who had a 1-0 record and a 4.37 ERA in six starts for the Storm Chasers.
"I've been in the bullpen a few times, so I've done it and know what it takes, so it shouldn't be that bad of a transition for me," Mazzaro said.
In the past two games, the Royals have been forced to use both of their long relievers, Luis Mendoza and Nate Adcock, when starters Luke Hochevar and Jonathan Sanchez were knocked out early in the game. Mazzaro, who last pitched on Thursday, would be available to provide multiple innings out of the 'pen against the Red Sox.
"We needed some more length in the bullpen," manager Ned Yost said. "We have expended our two long guys the last two days, and if something happened today unforeseen -- I think Danny Duffy's going to out and throw a good game -- you've always got to protect yourself in case of a line drive off a shin or something like that in the second inning. And we couldn't absorb that with no long guys."
Hottovy worked in five games with no record and a 3.38 ERA since being called up from Omaha on April 25.
"Hottovy did a nice job here, but it was a luxury, three left-handers," Yost said. "These guys have two [lefty hitters] on their club. There's two guys probably in the next two series for each team, so we have enough lefties to handle that. We needed some length."
Mazzaro, 25, was acquired from Oakland prior to the 2011 season in a trade that sent outfielder David DeJesus to the A's. Last season for the Royals, Mazzaro appeared in seven games (four starts) and had a 1-1 record and 8.26 ERA. He spent most of the year at Omaha and was 7-2 in 22 starts.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.