Doug Fister. (Photo by John McDonnell/The Washington Post)

As expected, the Nationals placed right-handed starter Doug Fister on the 15-day disabled list on Saturday afternoon with a strain in his right lat muscle, which means the two right-handers who were fighting for the final spot in the rotation, Tanner Roark and Taylor Jordan, will both be in the team’s opening day rotation.

Roark will start the Nationals’ home opener on April 4 against the Atlanta Braves and Stephen Strasburg will make his second start of the season, on regular rest, the following day. Jordan will make his first start of the season on April 6. Roark and Jordan both pitched so well in the spring that the Nationals found choosing between them difficult. But Fister’s recent injury created an opportunity for both.

“They’re both excited,” Manager Matt Williams said. “They’re both eager. I think they proved themselves in the spring. We created this competition between the two of them and they both responded. Those are very good signs.”

“It feels great, finally to get the answer, you know?” Roark added. “I’m very excited. At one point, it was like the middle of spring, I was just wanting to know. I had to just let it go. You can’t really worry about it. You got to go and do your job and get outs. ”

Williams said Saturday that Fister would be shut down for five days to rest his muscle strain. His 15-day disabled list stint is backdated to March 23. Because the right-hander was behind the rest of the pitching staff’s schedule thanks to elbow inflammation in spring, Fister might not return from this injury for at least three weeks.

Williams called Fister’s return “a little bit of a moving target” because it will hinge on the right-hander’s health and arm strength. Because Fister spent most of March recovering from elbow inflammation, he hasn’t yet thrown more than 47 pitches in a game this spring. He was scheduled to throw 60 pitches in a minor league start in Viera on Thursday but left after only 15 with discomfort in his side.

Williams mentioned three weeks to 30 days as possibilities Fister’s return. If his side feels better in five days, Fister can play catch and begin the traditional progression — throwing from 60 feet to 90 to 120 — before he throws from the mound again.

“The first thing is to take away the injury,” Fister said. “After that, the arm strength will come. Where I left a couple days ago, it felt like I had good arm strength. It was just a matter of building up pitch count. After we get the injury taken care of, it’s going to take a little bit of time to take care of the arm strength or the pitch count. It shouldn’t be too much.”

The Nationals nor Fister believe that the lat strain was a result of overcompensating for his elbow. “There’s no rhyme or reason for any of it,” Fister said. He said the lat discomfort was a “total surprise.”

The lat muscle, which runs inside of the armpit from near the shoulder to the side, is important in the deceleration of the arm, Williams said. Fister felt discomfort in there when he was warming up in the bullpen on Thursday and when he made a play in the field. He was removed from the game after the first inning after the tightness persisted.

“It’s more precaution than anything,” Williams said. “I don’t think it had anything to do with the elbow. The elbow has been fine thus far since he first had the inflammation.”

As Fister recovers, the Nationals will use both Roark and Jordan in the rotation, depth they lacked last season. Roark threw a longer-than-normal bullpen session on Saturday afternoon after the exhibition game against the Detroit Tigers was canceled. Jordan will return to Florida to throw in a minor league game on April 1 so he can stay on schedule for his first start of the season.

“I’m definitely happy to be here again, especially out of spring training,” Jordan said. “It’s something that I’m very proud of. Feels great to be here.”