Nate McLouth will undergo more tests on his inflamed right shoulder. (Photo by Jonathan Newton / The Washington Post)

As evidenced by the signing of Nate Schierholtz on Monday morning, the Nationals are concerned that Nate McLouth’s lingering shoulder trouble may, well, linger.

McLouth was eligible to return from the disabled list Sunday, but “isn’t improving as fast as he’d like to,” according to Manager Matt Williams. McLouth will undergo an arthrogram, an X-ray done with dye to better illuminate soft tissue components like ligaments, tendons, or cartilage.

“Once he gets those results, we’ll have a better sense of where he’s at,” Williams said.

The 32-year-old said last week the shoulder had been bothering him “awhile.”  The odd man out of the outfield mix when Bryce Harper returned from injury, McLouth  hasn’t played since Aug. 1. His last hit came June 28.

McLouth’s injury has limited Williams’s left-handed options off the bench to switch-hitters: Jose Lobaton and whichever of Danny Espinosa and Asdrubal Cabrera isn’t in the starting lineup. Since Cabrera was acquired from Cleveland on July 31, he and Espinosa had effectively been splitting time based on the starting pitcher: Espinosa has been starting against lefties, Cabrera against righties.

However, with a lefty toeing the rubber for Arizona on Monday night, Williams opted to play Cabrera, who owns a career average vs. left-handers of .279. The reason Williams gave was experience: as a recent American League transplant, Cabrera is the only player on the Nationals roster with major league at-bats against Vidal Nuno, who was traded to Arizona from the Yankees  in July in the Brandon McCarthy deal.

“It’s a balancing act with Danny, too,” Williams said. “Danny’s been really good against left-handed pitchers, and we want to keep him as fresh as possible when we’ve got an opportunity to get him in there against the lefties. But Asdrubal’s got experience. He’s got at-bats against [Nuno].”

>>> Like Cabrera, Mike Rizzo’s other recent acquisition, veteran reliever Matt Thornton, is settling in to Williams’s plans.

The 6-6 lefty is a well-known commodity in the American League, having pitched 652 games for AL teams since his debut with the Mariners in 2004. But he had yet to pitch in the National League until Aug. 6, after the Nationals claimed him off waivers from the Yankees.

Though interleague and shared scouting probably reduces the potential benefit Thornton would reap from the league switch, the move seems to suit him: he’s pitched 4.1 innings in a Nationals jersey, yielding three hits, walking no one, and striking out three.

So he’s earned Williams’s trust quickly. Aided by a few extra inning games, all five of his Nationals appearances have come in the eighth inning or later.

“He’s done it before. He’s pitched the eighth inning, he’s closed, he’s played in New York City so he understands that type of pressure,” Williams said. “It’s a little bit a function of the game and the types of games we’ve been in, but he’s not afraid to come in and face the heart of an order and shut down in the seventh, eighth, or ninth if that’s what he’s called to do.”

>>> Williams maintained after Rafael Soriano’s blown save Sunday that he will stick with Soriano as closer, but did admit that having three pitchers with closing experience — Thornton, Tyler Clippard, and Drew Storen — affords flexibility.

“It’s great that you have guys with experience doing it, if they get out there in that situation it’s not foreign to them,” Williams said. “We’ve talked about it a few times: there’s going to be a point in the season where you get to the late part of the bullpen and they’ve pitched a lot and you’ll have to make adjustments. We’ll see how Sori is today when he goes out and throws, and if we have to make an adjustment because he’s (pitched) in four or five, we can do that. We feel confident about that.”