The righty features a fastball thrown at three different angles, as well as a slider, curve and circle-change. He played three seasons for Sancti Spíritus in Cuba's Serie Nacional and for the island's 16-and-under and 18-and-under national teams.
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Unlike many Cuban players who must defect the island in order to establish residency outside of Cuba and the United States, the first step in becoming eligible to sign with a Major League team, Sierra was granted Spanish citizenship due to his grandfather's roots. He left Cuba legally, flying from Havana to Madrid in February.
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth.
This story was not subject to the approval of Major League Baseball or its clubs.