The move is retroactive to March 23. The Nationals made the announcement in finalizing their opening day roster after their scheduled exhibition game against the Detroit Tigers was canceled because of rain.
Fister will be shut down for five days before resuming his rehabilitation. The right-hander, obtained in an offseason trade with Detroit, is expected to miss anywhere from three weeks to 30 days, manager Matt Williams said.
Taylor Jordan takes over the No. 5 spot in Washington's rotation.
After a morning of light rain, with heavier rain forecast later in the afternoon, the game was called at its scheduled 2 p.m. start time.
Sanchez, slotted to pitch Thursday against the Royals, will throw a simulated game Sunday in Detroit, according to manager Brad Ausmus. Instead of the 80-85 pitches Sanchez was set to throw Saturday, he will likely throw 50 on Sunday in the simulated game.
Williams said Roark, who is slated to pitch the team's home opener Friday against the Braves, would throw a "heavy bullpen" on Saturday.