Bryce Harper hurt his thumb in Pittsburgh last September. Chaos ensued. (Charles LeClaire/USA Today)

The Nationals, as flawed as their bullpen may be, are back on the road this week with the best record in the National League — 24-13, 7½ games clear of their closest competition in the NL East, entering Monday’s games. Their offense has cooled off some but remains one of the best in the majors this season while the starting rotation continues posting quality outings.

Next up is a three-game series in Pittsburgh to face the stumbling Pirates (16-22), who sit in last place in the NL Central. The Pirates are coming off consecutive wins at Arizona but lost their previous six games.

Here are three things to keep an eye on as the Nationals make their only scheduled trip to PNC Park this season.

PIRATES’ OFFENSIVE STRUGGLES

The Pirates are six games under .500 because they have one of baseball’s worst offenses. Entering Monday, their 138 runs ranked 28th in the majors. Their .224 batting average ranked 29th, their .670 OPS ranked 27th, and their 80 wRC+ ranked 26th. (wRC+ stands for Weighted Runs Created Plus, a statistic that values each outcome depending on its impact while controlling park effects and the league’s current run environment. 100 is average. For comparison’s sake, the Yankees lead baseball with a 124 wRC+ and the Nationals are third with a 118.)

Any way you look it, with an old-school or new-school lens, the Pirates’ offense is lackluster. All-star outfielder Starling Marte’s 80-game suspension for performance-enhancing drug use hasn’t helped, but the rest of the vaunted outfield is falling short of expectations: Gregory Polanco has a 91 wRC+ and Andrew McCutchen’s start this season (83 wRC+) is even worse than last season, when his sudden decline was one of the sport’s top story lines.

As a result, Gerrit Cole, the Pirates’ ace, is a great example of why win-loss record is a faulty (at best) measure for starting pitchers: The right-hander has a 3.06 ERA but is 1-4 because the Pirates are averaging two runs in his eight starts. Meanwhile, they’re averaging 3.1 runs per start for Ivan Nova, who is 3-3 with a 2.48 ERA. The Nationals will encounter Cole but not Nova this week.

WHO’S NO. 5?

Another series, another TBA standing in as the Nationals’ fifth starter. This time it’s Wednesday, opposite the Pirates’ Cole.

A.J. Cole most recently occupied the spot after Joe Ross was sent to AAA, but he was optioned over the weekend to help make room on the roster for Shawn Kelley and Koda Glover. That probably leaves Washington with two options: Jacob Turner or Ross.

Turner tossed 11 pitches Saturday and 19 in the second game of Sunday’s doubleheader, but three days’ rest should be enough of a respite to take the ball Wednesday. The 25-year-old right-hander has pitched out of the bullpen recently, but impressed the Nationals with six innings of three-run ball at Coors Field last month and Manager Dusty Baker indicated his return to the rotation was in the cards.

If it isn’t, Ross seems like the other choice. Ross, who turns 24 on Sunday, struggled in his first start after his demotion but recovered to yield three runs over six innings in his second outing Friday. The Nationals instructed him to improve his stamina, further develop his change-up, and work on maintaining his arm slot, which they believe was the reason for a flat slider teams feasted on. Pitching coach Mike Maddux reported there was improvement beyond the numbers — namely in velocity — and the club wants him to work on maintaining his arm slot, which has been determined to be the reason for his slider.

ANY BAD BLOOD REMAINING?

The last time these teams met was for a three-game set in Pittsburgh last September, and it was eventful. The Nationals clinched the NL East after the second game only to receive a scare the next day when Bryce Harper hurt his finger sliding headfirst into third base in the third inning. But Harper didn’t slide because it was necessary; he slid because Pirates third baseman Jung Ho Kang faked applying a tag when there wasn’t a play. The Nationals deemed Kang’s actions dirty and voiced their opinions to the Pirates bench.

Then, in the bottom of the inning, A.J. Cole threw behind Kang. Cole was ejected, and benches cleared. He was later suspended five games, a ban that he completed this season.

Kang, however, won’t be around this week. He has been on the restricted list since March and could be for the entire season for a far more serious issue — he was arrested for his third DUI conviction since 2009 in his native South Korea and hasn’t left the country since. So perhaps there won’t be any lingering tension. But there’s always a chance. It’s baseball, and sometimes these things don’t die.

PROBABLE PITCHERS

Tuesday: RHP Stephen Strasburg (3-1, 3.28 ERA) vs. RHP Chad Kuhl (1-3, 5.81)
Wednesday: TBA vs. RHP Gerrit Cole (1-4, 3.06)
Thursday: RHP Tanner Roark (3-1, 3.88) vs. RHP Tyler Glasnow (1-3, 7.98)