Fernando Abad after allowing a homer to Justin Upton on Tuesday. (Jonathan Newton/The Washington Post)

After a midseason overhaul of the bullpen, the Nationals believed they had found the right mix of right-handers and left-handers. And for weeks, it worked. Ross Ohlendorf has been brilliant as a long reliever and spot starter. Left-handers Ian Krol and Fernando Abad had been dazzling. But in recent weeks, the bullpen, in particular the left side, has been a weakness.

“It was really good and now when we’re leaning on them, they’re having a few blips on the radar screen,” Manager Davey Johnson said. “One kid’s real young, still learning. [Krol] has a tendency to fly open and he just leaves the whirly bird down the middle, that’s what he did on [Jason Heyward on Wednesday]. But he’s been awfully good against them. Just made an inexperienced, bad mistake. Two strikes, can’t throw it down the middle.”

The Nationals bullpen posted a 3.51 ERA in the first half of the season, good for 14th in the major leagues. In the 19 games since the all-star break, the Nationals bullpen has a 4.97 ERA, the sixth-worst total in the majors. (To be fair, the entire pitching staff’s ERA in the second half is 4.53, also sixth-worst in baseball.)

The Nationals moved to strengthen a bullpen that was among the top third in the majors last season, and up to this point this year it is close to the bottom third. Overall, Nationals relievers have a collective 3.75 ERA, which ranks 19 out of 30 teams. A look at underlying numbers is revealing.

The Nationals are worst in the majors at stranding inherited runners. Washington’s relievers have inherited 88 runners and 36 of them have scored, a rate of 41 percent, last in baseball, and two percentage points worse than the next-worst Philadelphia Phillies. The major league average is 30 percent. The average major league team has handed their bullpen 154 runners on base — nearly double the Nationals’ total and a product of Johnson’s preference of giving relievers clean innings — but Washington has still struggled in fewer chances.

Johnson has often blamed himself for the management of the bullpen. He has, at times, called on the less obvious reliever in crucial spots. But there has also been a difference in talent. Before the season, Johnson stumped for a second left-handed reliever in the bullpen, which he didn’t receive until late May. General Manager Mike Rizzo has admitted the mistake in relying on one left-handed reliever to start the season. But there have also been blips in performance.

Right-hander Ryan Mattheus has a 8.44 ERA in six games since returning from the disabled list. Craig Stammen posted a 4.34 ERA through June and July. Because of the nature of the games played recently, Rafael Soriano, the $28-million closer signed this offseason, has pitched only twice in the past 12 days. Tyler Clippard has been the lone constant: a 1.99 ERA and 53 strikeouts in 49 2/3 innings.

The left side of the Nationals bullpen has stumbled of late. Albeit a small sample size, Abad has a 5.00 ERA in his past 10 outings. He has allowed five of the past eight inherited runners in that span to score. He has allowed runs in four of the past 10 appearances. He has also been used more often recently in less high-pressure situations; only eight of his 28 appearances have been when the Nationals are ahead or tied. Overall, he still has a 2.45 ERA but left-handers are now hitting .295 off him.

Krol, too, has slipped recently. Since the all-star break, Krol has a 5.06 ERA in eight outings and pushed his overall ERA to 2.66. In his 14 games before that, he had a 1.80 ERA. He has allowed all six inherited runners to score in the past eight games and has walked four batters in 5 1/3 innings. Despite the recent adjustments, Krol is holding left-handed batters to a .200 average. The rookie left-hander has found that hitters are reacting to his pitching differently than before, but it is now up to him to adjust.

“They seem to be jumping on my fastball a little more early than usual,” Krol said. “I’ve been trying to pitch backward, and that’s not my game. My game is to go after them with fastballs and attacking the zone, so I need to get back to what I was doing before and like I said, just clear my head and have a short memory.”