However, Romero has an electric arm with a fastball that averaged 96.1 mph in 2016, which ranked ninth among American League relievers who threw at least 40 innings, according to Fangraphs. He did have success at the beginning of last season when he set a Rays franchise record by retiring 17 consecutive batters, and he struck out 9.9 batters per nine innings last season.
That strikeout rate, high velocity and the fact that he is under team control until 2022 was enticing enough for the Nats to take a chance on a young pitcher after Tampa Bay needed to free up space on its 40-man roster upon signing first baseman Logan Morrison. Romero, who is out of options, brings the Nats' 40-man roster to 39, and he will likely enter competition with the remaining lefties in the Nationals bullpen.
Both Oliver Perez and Sammy Solis will return this year after playing key roles in last year's bullpen, and Matt Grace is the remaining left-handed reliever on the roster. Manager Dusty Baker grew fond of carrying three lefties through much of the first half last season, so perhaps there is room for Romero as well. His sample size in the Majors is small, but Romero has fared better against right-handers (.665 OPS) than left-handers (.880 OPS).
Washington parts with Rosa, 21, after two seasons in the organization following his signing as an undrafted free agent. He spent 2016 in the Gulf Coast League, and in 24 starts in the Minors the past two seasons, he posted a 3.83 ERA.
Jamal Collier covers the Nationals for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.