Matt Kemp’s days as a Gold Glove outfielder have passed, but after being traded to the Braves nearly three weeks ago and shifting from right to left field, he hadn’t been the defensive liability that some scouts (and Padres fans) predicted.

Until Thursday.

Kemp dropped a two-out fly ball on the warning track in the first inning, allowing two runs to score and giving the Nationals a lead they wouldn’t relinquish in an 8-2 win to start a four-game series at Turner Field. The Braves got a quality start from rookie Rob Whalen, but things deteriorated soon after their bullpen took over.

“I just dropped the ball,” Kemp said. “Unacceptable. It changed the mode of the game right there. Robbie pitched a great game. I didn’t help him out by dropping that ball.”

Whalen pitched six solid innings in his fourth major league start, and it was a one-run game until a couple of relievers stumbled again in a five-run eighth that raised the bullpen ERA in the past seven games to 8.25 with 21 walks.

“The eighth inning got away from us, kind of snowballed on everybody,” said interim manager Brian Snitker, whose Braves have lost five in a row and seven of eight games since winning six of seven.

They’ve lost nine of 10 against the Nationals this season and own a 6-23 record against Washington since the beginning of the 2015 season.

Braves shortstop prospect Dansby Swanson, after going 2-for-4 in his long-anticipated major league debut Wednesday, was 0-f0r-4 with two strikeouts and two pop-ups to second base. Remarkably, Swanson didn’t have a ground ball hit to him until his 16th inning in the majors, a slow roller that he fielded cleanly in the seventh inning Thursday.

The Braves trailed 3-2 before the eighth inning began with Arodys Vizcaino failing to retire any of four batters he faced in his first game back from a one-month stint on the disabled list. He gave up a leadoff single, then walked two before hitting Anthony Rendon with a pitch with bases loaded. That was all for Vizcaino.

“Hopefully if it was rust, he knocked it off and will be better next time,” Snitker said.

Vizcaino exited and rookie Mauricio Cabrera entered and promptly gave up a single and a bases-loaded walk before the first out was recorded in the inning.

What remained of an announced crowd of 24,099 – there weren’t nearly that many in attendance when the game began after a 1-hour, 16-minute rain delay – thinned considerably as the eighth inning wore on and incessant “whooo” cries came from fans scattered throughout the park.

Whalen had his best performance in his fourth major league start, allowing just three hits, three runs (one earned) and three walks with six strikeouts in six innings.

“He battled his rear off,” Snitker said. “I thought he did a really good job. That kid competes. He gives you everything he has.”

But Nationals rookie counterpart Reynaldo Lopez – like Whalen, just 22 and in his fourth start — had overpowering stuff, collecting a career-best 11 strikeouts with two walks in seven innings to beat the Braves and Whalen for the second time in six days.

“He threw me a 98-mile-an-hour slider down and away — accidentally,” Braves catcher Tyler Flowers said. “I mean, it was pretty good. If he can do that all the time, he’ll be a special one.”

Lopez (2-1) allowed four hits and two runs, one unearned after Kemp doubled in the fourth inning, advanced on a passed ball and scored on a Nick Markakis’ sacrifice fly to cut the lead to 2-1.

The Nationals pushed the lead back to two runs in the fifth on Jayson Werth’s two-out RBI double on a full-count slider, extending his on-base streak to 46 games to tie Rusty Staub’s franchise record in 1969-1970 for the Montreal Expos.

After failing to score following a leadoff walk in the fifth inning, the Braves got a run in the sixth when Freddie Freeman doubled, advanced on a wild pitch and scored on Kemp’s groundout to cut the lead to 3-2.

They also failed to score in the seventh after Flowers’ leadoff single, stranding two runners when Ender Inciarte grounded out to end the inning. Inciarte is 14-for-77 (.182) with runners in scoring position.

Lopez had eight strikeouts in the first three innings, striking out eight of the next nine batters after Adonis Garcia’s one-out single in the first. “He’s going to be something else,” Snitker said of Lopez, ranked the No. 46 overall prospect by Baseball America in its midseason list.

The hard-throwing right-hander limited the Braves to five hits and one run – on a Freeman homer – in seven innings of a 7-6 win on Saturday at Nationals Park, and Lopez handled Braves hitters with relative ease again Thursday night.