Nationals manager Matt Williams says he'll "push him [back] a couple of days, just to make sure; get it out of there."
Fister, acquired in an offseason trade with the Detroit Tigers, threw 30 pitches over two innings Sunday in his spring debut. Williams says Fister felt discomfort in his right elbow after the game.
He had an MRI on Thursday.
"We're not concerned about it," Williams said, but at the same time, they're "not going to take any chances."
The Nationals planned to start Tanner Roark against the Astros.