PHOENIX — The Washington Nationals did a lot to fix their bullpen in the past week, adding three arms at the trade deadline and making it so, in a perfect world, no one pitcher has to shoulder an outsize workload for the team to get by.

But that doesn’t mean all their problems have disappeared. A big one resurfaced Sunday — the unpredictability of Wander Suero — in a defeat that handed Washington a third consecutive series loss. The Nationals fell, 7-5, to the Arizona Diamondbacks after Suero gave up a two-run single to Adam Jones in the decisive seventh inning.

Starter Patrick Corbin wasn’t sharp in his first outing against his former team, giving up five earned runs in 5⅓ innings. The Nationals couldn’t handle Ketel Marte or Jones, who have beaten them in the past. But Suero’s rough inning — amid his up-and-down season — was what did them in. He hadn’t pitched since Wednesday. His ERA, shaved down by 9⅓ scoreless innings leading into this one, shot up to 4.80. The Nationals, in turn, have lost seven of their past 10 games.

Maybe the best part of their weekend in Arizona is that they don’t have to come back.

“We’ll get it back together,” Manager Dave Martinez said, firmly, when asked how the Nationals can end this recent slide. “We will.”

Corbin breezed through his first inning back in the desert, needing just nine pitches, before the Diamondbacks disrupted the reunion. Corbin had been dominant in his previous eight starts, posting a 1.76 ERA and 65 strikeouts in 51 innings. He had not allowed more than three runs in any of those outings. But the Diamondbacks were able to take him out of his weeks-long rhythm, just as they did with Stephen Strasburg in the second game of the series Saturday night.

It started with Corbin walking Christian Walker to lead off the second. Then Jones ripped an RBI double, Corbin walked Wilmer Flores, and Nick Ahmed brought home two more runs with a double of his own. The Nationals had grabbed an early lead on Gerardo Parra’s two-run homer in the top half of the inning. And then it was gone, like a breeze in a heat wave, and Marte stretched the lead with an inside-the-park home run in the next inning.

“I wish it went better today,” Corbin said of returning to where he spent the first six seasons of his major league career. “Just kind of never got in a rhythm out there. They had some good at-bats.”

After Corbin settled in for a bit, pitching two scoreless innings, the Nationals were able to work back into the game. Brian Dozier helped with a solo home run in the fourth. They evened the score in the sixth, against the Diamondbacks’ shaky bullpen, when Matt Adams singled with two outs, Dozier singled, Parra walked and Adams scored on a passed ball.

Martinez faced a tricky decision after the Diamondbacks loaded the bases again with an intentional walk. Corbin was due up after throwing only 70 pitches, and Martinez could have inserted a pinch hitter to try to break the game open. He instead stuck with Corbin, who grounded into a forceout, gave up another run-scoring double to Jones in the bottom half and exited with one out and runners on the corners.

To summarize, Martinez traded a critical out during a rally for, well, a Diamondbacks run, one more out from Corbin and a jam for new reliever Daniel Hudson. It didn’t seem like a fair deal in hindsight. It certainly wasn’t how the manager calculated it. But Hudson navigated his way out of trouble, stranding two runners in scoring position and giving the Nationals another chance in a tight game.

“His pitch count was down, and I liked him; he started settling down a little bit,” Martinez said of why he let Corbin hit. “The reason why we’re playing well is because our starters have done well.”

The Nationals knotted the score again with a quick rally in the seventh, but that only led to Suero’s latest struggle. He hit the first batter he faced despite an 0-2 count — he was trying to surprise Tim Locastro with a front-door cutter — then yielded a single to Marte. He got two outs to steady himself and set up an escape route before Jones whacked a single to the opposite field that put Arizona back on top.

Suero soon walked to the dugout with his eyes fixed on the Chase Field turf, his shoulders slumped a bit, his 53rd appearance having gone all wrong. The Nationals shuffled their bullpen by adding Hudson, Hunter Strickland and left-hander Roenis Elías, who is now on the 10-day injured list with a right hamstring strain. But Suero remains part of their plans, one of many high-leverage options, a pitcher they need to be consistent for the stretch run.

He had been pitching well and was well rested for a change, yet he didn’t have command of his cutter, the pitch he throws more than 70 percent of the time. He can’t be effective without it. And so he wasn’t, and the Nationals were pushed into another hole, and they headed to San Francisco frustrated, beaten up by injuries and with a bullpen that’s still — still — trying to figure it all out.

“They’d faced me before,” Suero said through a team interpreter. “So I think they were zoned in on the cutter.”

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