With their bullpen teetering for the first time this season, the Nationals kept Blake Treinen in the majors to provide a fresh, strong arm in relief and optioned Aaron Barrett to Class AAA Syracuse. The Nationals believe Treinen’s addition will prevent Barrett from fatiguing for the stretch run while granting Manager Matt Williams flexibility to use surging Ross Detwiler in a more prominent role.

The Nationals summoned Treinen from Class AAA Syracuse on Thursday night, ostensibly to hold a roster spot while newly acquired infielder Asdrubal Cabrera traveled to Washington from Cleveland. But Treinen will remain with the Nationals to bolster a bullpen that has amassed a 4.87 ERA over the past month. Barrett will head back to the minors to make room.

Barrett has been a strong presence in the Nationals’ bullpen in his rookie season, dominating right-handed hitters in his wicked slider. But lately he has shown fatigue. In July, Barrett posted a 9.45 ERA. The Nationals had looked in the trade market for a reliever to bolster their bullpen, which has been healthy all season.

Barrett’s struggles coincided with the balk umpire Joe West called on him July 1. Barrett executed his usual motion, the same exaggerated set as always, but West called him for a balk. Barrett tinkered with his delivery and tried not to make a significant change, but even the subtle difference may have affected him.

“I don’t think it helps, let me put it that way,” Williams said. “We’re all creatures of habit and we’re all creatures of timing in this game. If you have your comfort level up and your timing right, everything will work out. When something like that is put into the mix, it disrupts that. The fact of the matter is, regardless of any of that, if you don’t get your slider where you want to get it to, or your fastball where you want to get it to, you tend to give up base hits. But it was certainly something different for him.”

The Nationals want Barrett to regain his mechanics in a “structured” atmosphere at Class AAA, Williams said. The innings will be less taxing for Barrett, who has frequently pitched in high-leverage situations against some of the league’s most fearsome hitters. (Among the batters he’s faced multiple times: Giancarlo Stanton, Buster Posey, Andrew McCutchen and Mike Trout.) The Nationals want him to remain effective for September and, they hope, October.

“Yes, exactly,” Williams said. “Again, we never want to lose him. But you try to pick the right opportunity to do so, and we felt this was the right opportunity. Blake’s got experience here, can provide us that long situation if we need it. And certainly can come in for one inning, too. It provides Aaron the opportunity to work on some things, too.”

Treinen has been starter at Class AAA Syracuse, where he has gone 7-1 with a 3.09 ERA in 13 starts. In the majors this season as both a long reliever and rotation fill-in, Treinen owns a 2.29 ERA over 35 1/3 innings. Treinen throws a heavy, mid-90s sinker, which he uses to limit home runs and induce gobs of groundballs.

“I’m ready for it,” Treinen said. “I’m excited for the opportunity. Obviously, there’s two sides to every story. It sucks, because [Barrett] is a good friend of mine. But I’m excited for the opportunity to do the best I can to help this team. I don’t want to be a void. I want to do my job and help them in the race they’re in.”

Treinen will serve as the Nationals’ long reliever, a right-hander Williams can hold back for extra innings in a tight game. Detwiler had filled that role. Now, the Nationals can use him in more high-leverage spots at a moment he’s becoming the pitcher they envisioned when they moved him to the bullpen in spring training. In the past two months, Detwiler has a 2.08 ERA and a 0.84 WHIP over 21 2/3 innings.

“It also may provide Stammen the same opportunity, to come in and pitch in the sixth to get us out of the sixth and not have to worry about if the score stays tied and we go extras,” Williams said. “It’s important for us.”