PHOENIX — This was supposed to be the start of something, something fresh and new, the Washington Nationals’ first chance to showcase the bullpen they rebuilt at the trade deadline.

And, for the most part, it was. But the lasting image of Friday night’s game, a 3-0 win over the Diamondbacks, may be Roenis Elías clutching the back of his right leg as he ran toward first base in the seventh inning. The Nationals acquired Elías on Wednesday, as one of three relievers they added during a furious 30-minute stretch, as a lefty who could help sure up their bullpen for the pennant race.

His debut started well, once he retired the first two batters he faced in a Washington uniform. Then it ended with an at-bat, an injury, and uncertainty of what comes next. The Nationals were treating it as a cramp after the game, according to Manager Dave Martinez, but Elías later mentioned “pulling” his hamstring. He will be reevaluated at Chase Field on Saturday.

“I felt a small pull in the back. I can’t tell you if it was the hamstring or a cramp,” Elías said in Spanish through a team interpreter following the win. “I just felt it and immediately decided to shut it down. That’s why they told me they’ll look at it tomorrow more in depth. I can’t give you any details as to what it is.”

Elías curiously stepped into the on-deck circle once the top of the seventh began. Relievers don’t typically hit — and Elías had just six prior at-bats in his career — but Martinez wanted him to face lefty Jake Lamb in the bottom half of the inning. Martinez was already down to three bench players, with Howie Kendrick unavailable due to a leg cramp, and so he sent Elías to the plate.

Martinez instructed Elías to keep the bat on his shoulder. But Elías didn’t listen. He hit a high bouncer over the mound, worked into a sprint, then slowed a step from the base. He then limped off the field, slowly, and was checked out by athletic trainer Paul Lessard in the dugout. While he went to the trainer’s room, the Nationals’ bullpen pieced together the rest of its best performance of the season. Joe Ross had allowed Arizona’s lone hit, to starter Alex Young, in 5 ⅓ innings. Elías got his two outs without a hitch. Then Hunter Strickland, another new addition, pitched a perfect seventh, Fernando Rodney matched that in the eighth, and Sean Doolittle did the same with a one-two-three ninth.

Washington, in turn, inched back to within six games of the Atlanta Braves in the National League East. But the victory may come with a painful price.

“He was told not to swing, about as many times as I could tell him in Spanish and English,” Martinez said of Elías. “But he’s competitive, you know?”

“First of all, Davey told me not to swing,” was how Elías began his postgame interview with a small group of reporters. “But being an athlete, and you’re a competitor out there, I figured I’d try to put the ball in play. Once I saw the ball, I thought there was a good chance for me to beat it out.”

The Nationals woke up Friday in a three-way tie atop the National League wild-card standings, level with the Philadelphia Phillies and Chicago Cubs, looking toward a tight pennant race. The Nationals have been chasing for weeks now, with the Atlanta Braves as their primary target, missing a few chances to make up ground in the division. Their season has been one long upward climb. And their work is far from finished.

Yet here, in Phoenix, the Nationals are being chased for a change. The Diamondbacks entered this series 3½ games back of Washington and Philadelphia for the second wild card spot. They were the ones doing the simple math, calculating what they needed this weekend, how they could perhaps shift their playoff hopes with a couple wins.

In Arizona’s way, at first, was Ross in the Nationals’ fifth starter spot. Washington improved its bullpen at Wednesday’s trade deadline, acquiring Daniel Hudson from the Toronto Blue Jays, and Strickland and Elias from the Seattle Mariners. But efforts to add a starter fell short, leaving Ross, Erick Fedde and Austin Voth the potential solutions to a rotation depth issue.

Ross took a big step forward Friday, at least eventually, even though his command was shaky at the start. The 26-year-old righty walked five of the first 11 batters he faced. But he escaped the second with a well-placed sinker, inducing an inning-ending double play, and ended the third with a great barehanded play of his own. His outing finished one out into the sixth, at 87 pitches, and that’s when Elías jogged in from the bullpen to protect a two-run lead.

The 31-year-old lefty was crisp right away, getting David Peralta to fly out and striking out Adam Jones on a low breaking ball. But danger lurked once he was told to grab a helmet and bat. He never made it back out to face Lamb in the seventh. Strickland handled the inning instead, and Rodney breezed through the eighth, and almost everything was good and easy for the Nationals’ revamped bullpen.

The caveat was Elías reaching for his leg mid-stride. The takeaway, on a night meant for turning the page, was a gamble gone wrong.

“I’m not that worried about it. I feel pretty good. I can walk around. It doesn’t seem like it hinders me too much to walk around,” Elías said. “We don’t know yet the extent of the injury, but I’m positive it’s not anything serious.

“And a few days out, I might be ready to go.”

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