The move for Rivero is retroactive to April 18, the team said before Wednesday's game with the Cardinals. Rivero made his major league debut last Friday, giving up one earned run and three hits in the ninth inning of a victory over the Phillies.
Grace has yet to pitch in the major leagues. Last year, Grace posted a 5-1 record with a 1.17 ERA in 50 games total with Double-A Harrisburg and Triple-A Syracuse.
Washington selected him in the eighth round of the 2010 draft.