Manager Matt Williams said that Fister will be evaluated in the coming days and have his throwing schedule pushed back, but that the team doesn't believe the injury is anything serious.
With Fister sidelined, right-hander Tanner Roark will make today's start for the Nationals.
"We had an MRI taken yesterday and it shows a little inflammation in there, so we're gonna push him a couple days, just to make sure," Williams said. "Get it out of there. Came back good, he's just got some inflammation.
"We're not concerned about it. He feels like if (it was the regular) season he could go, he could pitch. But we're not gonna take any chances. We'll push him a couple days, redo the pitching schedules and just make sure he feels good next time out."
Fister, who was acquired by the Nats this winter in a trade with Tigers, made his Nationals spring debut on Sunday against the Marlins. He went two innings, allowing a run on two hits with a walk and two strikeouts, but complained about some issues with the elbow after the appearance.
"After his start, he started feeling a little discomfort, a little tight in there," Williams said. "So that's why we got the MRI, just to take a look. Reports are good, but we're just gonna err on the side of caution right now and make sure he's good."
Roark was scheduled to pitch today anyway, so the Nats will just bump him up and have him get the start in Fister's place.
Meanwhile, Williams said that right-hander Ross Ohlendorf, who left last night's game against the Braves with lower back spasms, is feeling better this morning, but it appears that the veteran righty will likely be sidelined for at least a couple of days.
Ohlendorf faced four hitters last night and didn't retire any of them. He walked off the field with a trainer after his spring debut was cut short.
"He's better today, but he had a pretty good spasm last night," Williams said. "So we'll continue to treat him and see where he lands next time out. But right now, we're just treating him, making sure he gets back to healthy before he goes again. ...
"We don't expect it to be anything structural, he just had a cramp, back spasm. So we'll see how it goes."