WASHINGTON -- The Nationals have been so encouraged by the start of the season because two areas of their team are working so smoothly. Their starting pitching staff has been stellar, including an excellent outing from left-hander Patrick Corbin on Friday, and their lineup has been clicking, paced by a red-hot start from Anthony Rendon at the plate.

And yet this bullpen has been their downfall so often through the early part of this season, most recently in Friday night’s 6-3 loss to the Pirates at Nationals Park. Nats relievers began the day with the highest ERA in all of baseball before they gave back a one-run advantage in the eighth inning, followed by Justin Miller surrendering a three-run homer to Colin Moran in the 10th to seal a frustrating loss in the series opener.

“We got to iron out that bullpen, we got to iron out the back end of the games,” manager Dave Martinez said. “We can’t run [Sean] Doolittle out there for six outs, five outs, every day. These guys understand that and... just today, executing pitches just wasn’t there.”

Corbin was excellent on Friday, tossing seven innings of one-run ball with 11 strikeouts and handing the Nationals a one-run lead in the eighth inning. But the combination of Tony Sipp and Kyle Barraclough could not hold on. Sipp gave up a pair of hits before being relieved by Barraclough, who allowed a two-run single to Starling Marte.

Barraclough spent most of last year as the closer for Miami and inherited only two runners all season, but he has found himself in those situations more often with the Nats, inheriting seven runners on the bases already this season, all of whom have scored.

“You come in and you’re attacking guys,” Barraclough said. “You’re not coming in and trying to be tentative. You’re going right at them. I need to execute a little better.”

The Nats were nearly saved by Rendon, who mashed a pair of home runs, a solo shot in the third inning and then a game-tying blast in the eighth. It extended Rendon’s hitting streak to 11 games on the year and was the ninth consecutive game in which he has recorded an extra-base hit, matching a club record. He is now batting .426/.481/.957 to start the year.

But in the 10th, Matt Grace threw three pitches and left with runners at the corners for Miller, who left an 0-2 fastball over the heart of the plate for Moran’s three-run homer.

After the game, Miller was diagnosed by team doctors with a lower-back strain, which he says has been bothering him since last weekend in New York. Miller, who has now allowed four homers in his last three games, had been receiving treatment from the team training staff, but after Friday’s outing the Nats decided it was time to sit him down instead of letting him pitch through it. He will be placed on the 10-day injured list Saturday.

“It just feels like somebody's punching me in the kidney every single time I try and throw a ball,” Miller said. “It just feels like something's there that's not allowing my body to get through it.”

Now, the Nationals will have to find a new arm to bolster a bullpen that has recorded an ERA of 8.12.

“Davey instilled to us the last couple years, ‘We’re a family, and we got to stick together,’” Rendon said. “We talked about that as a team prior to the season starting. We know we’re going to go through great spells where we’re going to win 10, 15, 20 games in a row and we’re going to go through spells where we’re going to lose 5, 10 or you’re going to lose 8 out of 9, whatever it might be. A part of our team is struggling at the moment, so we need to just be there as a family and continue to support each other.”