Grace, 26, went 2-1 with a 5.25 ERA in 12 innings over 17 appearances. He allowed four runs and three hits without recording an out in Washington's 8-2 loss at Cincinnati on Sunday.
Rivero, 23, pitched one inning during his big league debut on April 17, allowing one run. He was placed on the 15-day disabled list on April 22 before being assigned to Syracuse. In eight games at Triple-A, Rivero went 0-2 with a 6.75 ERA.
Walters, 30, was assigned to Syracuse. The right-hander made five starts between Double-A Tulsa and Triple-A Oklahoma City this season.
Washington opens a three-game interleague series at home against Toronto on Monday.