To make room on the 40-man roster, Kansas City placed right-handed pitcher Henry Barrera on the 60-day disabled list. He is recovering from reconstructive surgery on his right elbow last June 30.
Hernandez, a 23-year-old right-hander, made 26 starts last season for Triple-A Tacoma, posting a 10-9 record and 5.23 ERA. The Seattle Mariners placed him on waivers and he was claimed by the Red Sox on Feb. 10, but the Sox designated him for assignment Monday.
"He's a young guy who's pitching at a high level for his age," Royals general manager Dayton Moore said. "He's got a solid arsenal of pitches, he's very athletic and young. He has two [Minor League] options remaining and he gives us more depth. Steve Foster, our bullpen coach, had him in Florida and he liked him, and our scouts liked him as well."
Hernandez, who has not pitched in the Majors, has a 46-45 record and 4.27 ERA in six years in the Minors. He was a third-round Draft choice of the New York Mets in 2004 and was traded to the Florida Marlins for catcher Paul Lo Duca in '05. The Marlins dealt him to the Mariners for pitcher Arthur Rhodes in '08.
Barrera pitched in just four games last season for Class A Wilmington before having surgery.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.