The Nationals plan to call up reliever Abel de los Santos from Class AA Harrisburg on Monday, according to a source with knowledge of the transaction, adding the hard-throwing right-hander to their bullpen.
It's not known yet who de los Santos will replace, but the Nationals will have to clear a spot on their 40-man roster before they can officially purchase the reliever's contract and bring him to the major leagues.
De Los Santos, 22, has impressed in his first season in the Nationals organization, posting a 2.65 ERA in 26 appearances with Harrisburg, striking out 35 batters while walking only seven.
The Dominican native was acquired from the Rangers in December, along with infielder Chris Bostick, for left-hander Ross Detwiler, who just last week was released by Texas after refusing a minor-league assignment. Originally a starter at the lower levels of the Rangers' system, de los Santos was converted to a reliever in 2013 and saw his career take off.
"Since he's gone to the bullpen, his velocity and stuff has spiked, and he had a terrific year last year," Nationals general manager Mike Rizzo said after making the trade. "We feel he's the guy that could come relatively quickly."
De Los Santos will become the 24th different pitcher used by the Nationals this season, the seventh to make his big-league debut.