(Photo by John McDonnell/The Washington Post)

Nationals right-hander Doug Fister, the prized offseason addition who gave them perhaps the best starting rotation in baseball, exited a minor league start after one inning Thursday with a right lat strain. Already trying to build arm strength in time for the regular season in the wake of early-spring elbow inflammation, Fister can almost certainly be ruled out to make the first scheduled start of his Nationals tenure.

“It certainly is a setback,” Manager Matt Williams said.

Nationals team physician Wiemi Douoguih will evaluate Fister on Friday in Washington. The Nationals do not know how much time he will miss. Even if he suffered only a slight setback, it will prevent him from building sufficient arm strength to make his first start, which had been projected for April 6.

Both Tanner Roark and Taylor Jordan, who had been vying for the final spot after making strong-yet-brief impressions as rookies in 2013, will likely make the Opening Day rotation.

“Any time you have to shut anybody down, you’re always concerned,” pitching coach Steve McCatty said. “If it’s something or because his elbow was bothering him, I would have shut it down over that, too. You got to be cautious about something like this.”

Fister had found optimism at every step of his recovery from elbow inflammation, which developed after he made his first spring training start March 2. He returned to major league action Saturday and dominated the Marlins for 3 2/3 innings. He needed to clear two more hurdles – 60 pitches in a minor league game Thursday, and another 80 pitches five days later – and he would take his place in the rotation.

Before Fister took the mound on a backfield in Viera, McCatty asked him how his elbow felt. Fister replied that he had developed tightness in his lat, a muscle that runs from arm pit to waist. It adducts a pitcher’s shoulder, making it of crucial importance. If Fister had felt the problem before, he had not mentioned it.

Fister threw 15 pitches over one inning. Watching from behind the catcher, through a chain-link fence, McCatty observed nothing wrong. Fister’s sinker darted downward. His velocity seemed normal. But when McCatty asked Fister if the lat had loosened, Fister replied, “No.”

“I said, ‘I really don’t want to do this, but if you feel something in there, I really would like to shut this off,’ ” McCatty said. “He shook his head and agreed. He felt a little tugging in there. So we stopped.”

Williams did not rule out Fister making his first start. “We’ll see what the doc says,” Williams said. “It’s a little early to tell right now.” But other Nationals starters have thrown at least 80 pitches in an outing. Fister has not thrown more than 47 in one session all spring.

“I was not going to let him attempt to get through,” McCatty said. “If I told him he was alright to go, he probably would have went out there. You got to be smart in those situations.”

The Nationals already realize they will be probably be without Fister, a 6-foot-8 sinkerballer who ranked as one baseball’s 10 best pitchers according to some advanced metrics, for the first two weeks of the season. Now, they will wait and hope it will be no longer.

“We don’t know,” Williams said. “He came back fairly quickly from the elbow inflammation. We’ll see. It’s a little early to tell right now.”

The Nationals had debated all spring between Roark and Jordan, two right-handers who impressed in major league cameos last season. They would feel comfortable with either in their rotation. They waited to decide in case Fister suffered a setback. Now, it seems, both will be in the rotation.

“They both earned the right to pitch,” Williams said. “We’ll have to see how he feels tomorrow, what the doctor says, where we’re at. Right now, we just know he couldn’t make it through his start today. We’ll see where we’re at tomorrow.”

In a trade widely viewed across baseball as a major win, the Nationals acquired Fister from the Detroit Tigers this winter for left-handed reliever Ian Krol, utility infielder Steve Lombardozzi and pitching prospect Robbie Ray.