Washington Nationals Manager Matt Williams checks the line up before a spring training game earlier this month. (John McDonnell/The Washington Post)

Matt Williams developed acid reflux sometime in the last week. He stays awake all night. The Washington Nationals’ roster provides him fun and frustration. It wracks his nerves and stimulates his mind. Decisions he will need to make soon occupy him. They are decisions he never needed to make before. “When you’re the infield coach, man, you just go bang fungoes,” Williams said.

In the last week of spring training before his first opening day as a manager, Williams must choose the final spots of his first 25-man roster. The initial version of the Nationals seems settled aside from three positions. They need to determine their fifth starter, which could in turn determine the last reliever in their bullpen, which may indirectly be tied to the final available bench spot.

The Nationals will fly to Washington on Thursday after their final Grapefruit League game. Williams plans to finalize the roster before players board the plane. For the next three days, Williams will grapple with a new pressure. He will be the one who tells players they did not make the team, and he will be the one who most shapes the complexion of a major league team.

“It’s been fun, though,” Williams said. “It’s a fantastic learning experience. All of it is. But to be one of the real voices in our 25-man roster construction, what we want on our bench and how we’re going to go about doing it, it’s a pleasure to do. It’s a lot of fun. We have to make the right decisions, though.”

The choices will be tough. The Nationals will send talented to players to Class AAA Syracuse and risk losing others. Tyler Moore, a key part of the bench for two years, will likely survive the opening day cut only if 12-year veteran Jamey Carroll does not. Tanner Roark, who punched up a 1.51 ERA in a fall cameo, may not have a spot in the rotation. Scouts have buzzed about Blake Treinen’s electric right arm all spring, and he is not even on the 40-man roster.

“They got some good problems to deal with,” one National League scout said.

As Williams deals with them, players on the bubble wait and wonder. In a week, they might line up along a baseline at Citi Field and tip their cap. Or they might be looking for an apartment in Syracuse. Or, in the case of veterans, they might be getting used to a new organization.

“It’s just the anticipation factor, I guess,” Roark said. “Just wanting to know. But it’s not up to me. It’s out of my hands. So it’s just going out there every time they give me the ball, go out there and compete.”

The most essential decision concerns the rotation: Roark or Taylor Jordan? Last season, Roark, 27, began his late-season call-up as an unknown commodity. He went 7-1 with a 1.51 ERA swinging between the bullpen and the rotation. Coaches raved about his cool, competitive demeanor, precise command and advanced feel for pitching.

Jordan, 25, rocketed from Class A Potomac to the majors last season and made nine starts before the Nationals shut him down as part of his protocol for recovery from Tommy John surgery. He thrived with a vicious sinker that produces loads of groundballs. This spring, improvements to his offspeed pitches have led to increased whiffs — he has 18 strikeouts and one walk in 15 innings.

Towering right-hander Chris Young ostensibly remains in the competition, but Sunday morning Williams spoke of the decision in binary terms. “They both bring fantastic opportunity for our club,” he said, referring to Roark and Jordan. “There’s track record on both sides.” Young, 33, can opt out of his minor league contract Thursday if he is not on the major league roster.

On Sunday afternoon, Roark made his final spring training start, allowing one run on two hits over 52 / 3 innings against the New York Mets. In 132 / 3 major league innings, Roark allowed five earned runs, 11 hits, three walks and struck out 11.

“I think he’s made a very good case,” Williams said. “We’ll get a good look at Taylor again. But Tanner certainly made a really strong case.”

Jordan will conclude his spring training Monday in a start against the Marlins. Their final impressions, though, will hardly factor in Williams’s evaluation.

“I wouldn’t anticipate there’s a whole lot that they can do from a performance standpoint that would sway either way,” Williams said. “Want to get him through it healthy. That’s what we want.”

The fifth starter will, coincidentally, determine who starts the Nationals’ home opener April 4. He will actually pitch the fourth game of the season — the first at Nationals Park — because the Nationals want to give Doug Fister extra days to recover from elbow inflammation.

Most rival evaluators view the decision as a toss-up. Even some who see Roark as the better pitcher believe Jordan may earn the nod. If Jordan wins the spot, Roark likely would make the team as a reliever. If Roark is named the fifth starter, Jordan would be sent to Class AAA for more seasoning — he is younger and, unlike Roark, has no experience pitching in relief. And so Jordan becoming the fifth starter would mean keeping two of the Nationals’ best arms in the majors.

Should Roark claim the fifth spot and leave an open bullpen position, Williams would have another decision. Treinen, a non-roster invitee without experience above Class AA Harrisburg, forced his way into the mix with his 97-mph sinker. Aaron Barrett, Harrisburg’s closer last season, throws a slider worthy of the bullpen’s back end and has not allowed an earned run in eight spring games. Left-handers Michael Gonzalez and Xavier Cedeno are also receiving consideration.

The Nationals’ bench likely will consist of Jose Lobaton , Nate McLouth , Danny Espinosa , Scott Hairston and either Moore or Carroll. Hairston’s spot, seemingly assured at the start of spring, fell into question this week when Williams declined to commit to him making the team. Hairston, whom the Nationals traded for last season, will make $2.5 million guaranteed.