Milwaukee’s Scooter Gennett hits an RBI single for the go-ahead run in the eighth inning of the Brewers’ 6-5 win against the Nationals. (Morry Gash/Associated Press)

The seventh inning has proved difficult for the Washington Nationals all season. An inconsistent bullpen has created the problem, and this six-game trip has underscored its urgency. They have taken leads into the seventh in their past three games and watched the margin slip away each time. In a 6-5 loss to the Milwaukee Brewers on Thursday, the collapse started in the seventh and continued into the eighth.

An overworked bullpen and the lack of lock-down relief options have forced Manager Matt Williams to lean on his starters. As the Nationals nursed a one-run lead in the seventh inning Thursday, Williams stayed with Tanner Roark, who struggled much of the night with his command. Roark got one out and then gave up a game-tying home run to Gerardo Parra. An inning later, the Brewers got the decisive run thanks to a wild pitch from Aaron Barrett that skipped past catcher Wilson Ramos, a throwing error by Anthony Rendon and a two-out single from Scooter Gennett.

Adding to the concern: An already-banged up bullpen suffered another loss when Barrett, who started the eighth, left the mound with an apparent injury.

The defeat was the Nationals’ 10th in 13 games. Coupled with the Mets’ victory, the loss knocked the Nationals (31-29) out of first place, a half-game behind New York.

“In the late innings, any mistake is magnified,” Williams said. “Ball got by Wilson. Guy got on first base, and we threw the ball away and got a guy in scoring position, and they got a base hit. Late in games that’s going to hurt you. We need to make sure we’re providing ourselves more opportunities to score runs late.”

When Nationals pitcher Stephen Strasburg returns from the disabled list, will he be back to his strong form or will he get stuck in his head again? (Post Sports Live/The Washington Post)

The injuries are growing. Jayson Werth, Stephen Strasburg, Doug Fister and Ryan Zimmerman — among others — are on the disabled list. Head down, Barrett left the game with a trainer after firing two 92-mph fastballs outside the strike zone, a few ticks lower than normal for him.

Immediately after the game, Williams said the team didn’t have an update yet on Barrett, who was being evaluated by medical staff. Williams said they checked on Barrett mid-at-bat because “he was uncomfortable” on the mound.

A game that began well for the Nationals went awry late. Clint Robinson, one of the players who has received a larger role with the injuries, powered the Nationals’ offense with his first major league home run in the second inning off Brewers starter Matt Garza.

“It felt great,” Robinson said. “Everything you dream of.”

But the lead evaporated. Roark struggled to keep pitches down in the strike zone, and although the Nationals kept giving him leads, he was unable to hold them. Denard Span led off the game with a double and scored on Bryce Harper’s single and an error by shortstop Jean Segura. But Roark gave the lead right back in the bottom of the first when Parra scored on Ryan Braun’s triple, a blast that came on a fastball left over the plate.

The Nationals again staked Roark to a lead, this time a 5-1 margin in the fourth inning. With Robinson and Michael A. Taylor on base after singles, Span hit a ball up the middle. The ball bounced over Segura’s glove, and Robinson scored. Parra’s wild throw home put two in scoring position for Rendon with two outs.

Rendon hit a grounder to third baseman Aramis Ramirez; he fielded the ball and had plenty of time to throw out Rendon at first base. Instead, he skipped the throw and first baseman Adam Lind couldn’t handle it. It was the Brewers’ third error of the game.

Slowly, the Brewers chipped away at Roark and his inconsistent command. Two groundouts in the fourth scored two runs after Roark yielded a single to Braun and a double to Lind, whittling the Nationals’ lead to 5-3. Roark allowed another run in the fifth, capped by Lind’s RBI single.

“That’s on my shoulders for not keeping it there and letting them back in the game,” Roark said.

Based on the way the Brewers hit the ball off Roark in the sixth inning — two flyouts to the warning track — it seemed possible it was his final inning. But Williams sent Roark back out to the mound for the seventh at 94 pitches. The leash, however, wasn’t long as relievers Felipe Rivero and Barrett were warming up.

“Our bullpen is heavy,” Williams said. “We want to make sure that he can get through that inning if possible. Unfortunately he hung the change-up to Parra. We were going to get him for Braun anyway, but he never got through Parra.”

Roark got the inning’s first out but then misplaced a change-up to Parra, who slammed it over the right-center field wall to tie the game.

“I love that [Williams] trusts us and trusts me,” Roark said. “That gives me more fire. It was my last inning. Just bad pitches on my part. Pitch selection and not throwing what I should have thrown.”

With Roark out of the game, Rivero finished the inning. But in the eighth came Barrett, who has logged the heaviest workload of any Nationals reliever this season. Thursday was his 30th appearance in the Nationals’ 60 games. He struck out Ramirez to start the inning, but Ramirez reached when the ball got away from Ramos.

Barrett then got Shane Peterson to hit what appeared to be a double play ball. But Rendon’s throw skipped past Robinson. A groundout moved Peterson to third base. A high slider from Barrett to Gennett led to a single down the left field line. A batter later, Barrett was out of the game.