Granillo has claimed a spot in the Nationals' Opening Day bullpen, Jessica Camerato and Paige Leckie of MLB.com report.
The 25-year-old right-hander made his MLB debut last season with the Cardinals, posting a 4.71 ERA, 1.38 WHIP and 18:7 K:BB over 21 innings, before being traded to the Nationals early in February. Granillo is expected to fill a low-leverage role to begin his tenure with Washington.