Rivero, 23, is ranked No. 16 in Washington's Minor League system by MLB.com's Prospect Watch. In five Grapefruit League games this spring, he went 1-0 while allowing just four hits in 5 2/3 scoreless innings and striking out six. Rivero also made three appearances for Syracuse before his callup, allowing three runs in 2 2/3 innings.
Across three levels of the Minors last year, Rivero struck out 50 batters while walking just 19 in 53 2/3 innings. He went 2-7 with a 3.35 ERA but worked exclusively as a starter.
Rivero was acquired in a Feb. 13, 2014, deal with the Rays that sent right-hander Nathan Karns to Tampa Bay.
Joey Nowak is a reporter for MLB.com. Follow him on Twitter at @joeynowak. This story was not subject to the approval of Major League Baseball or its clubs.