Fister, 30, missed time earlier in camp with elbow inflammation, though the team says the elbow and lat strain are unrelated. He was scheduled to return to the rotation in the second week of the season, but that seems unlikely now.
Acquired from the Tigers in an offseason, Fister went 14-9 with a 3.67 ERA (115 ERA+) in 32 starts (and one relief appearance) and 208 2/3 innings for Detroit last season. He had a 128 ERA+ in parts of three seasons with the Tigers.
With Fister out, the Nationals will open the season with Stephen Strasburg, Gio Gonzalez, Jordan Zimmermann, Taylor Jordan and Tanner Roark in the rotation.