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.