Barrios was not ranked by MLB.com as one of the Pirates' top 30 prospects, but has posted a 3.12 ERA and 27 saves while advancing from the rookie Gulf Coast League all the way to Triple-A Indianapolis in two-plus seasons since converting from third base.
"Good arm," said Brewers general manager Doug Melvin. "He shot through the system pretty quick to Triple-A, but we're going to send him to [Double-A] Biloxi. He needs some work with his delivery, just because he's only pitched 100 innings.
"He sits 96-97 [mph] and we do like the arm strength. We do know there's work to be done, too, to get him consistent. He needs to come out and throw more breaking balls, more change-ups."
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Melvin mentioned other former position players who found success after switching to relief, including the Cubs' Jason Motte and Dodgers' Kenley Jansen.
"There's guys out there who have good, live arms who converted," Melvin said, "but it just takes a little bit of time."
Said Pirates GM Neal Huntington: "We gave up a really good young arm that has velocity, that has the makings of off-speed pitches, that hasn't been pitching all that long. Milwaukee did a nice job. They got a guy that has some ceiling. When we met their ask, the deal came together."
Adam McCalvy is a reporter for MLB.com. Follow him on Twitter @AdamMcCalvy, like him on Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.