Stephen Strasburg left Sunday’s game after four innings with discomfort in his upper back. (Katherine Frey/The Washington Post)

It’s the starting pitching, stupid. That’s the Washington Nationals’ biggest problem of 2015.

At first glance, this sounds unlikely, bordering on ridiculous. The Nats’ problem is massive injuries. Or the manager. Or the shaky bullpen bridge to the closer. Or all the errors. They don’t score enough runs. They don’t hold enough leads. Everybody knows it. The Nats sometimes think it themselves. But it’s not true.

A lot of us have been big dummies in spotting this overarching trend. The grand expensive overkill Nats rotation — the “strength that was strengthened,” according to the team when Max Scherzer was signed last winter — was supposed to be the strongest element of any team in the entire sport. It was so overloaded that the Nats couldn’t find a spot for 15-game winner Tanner Roark. Prepare to behold greatness.

Yet this rotation of Scherzer, Jordan Zimmermann, Stephen Strasburg, Doug Fister and Gio Gonzalez has turned out to be the core reason Washington is 5 1/2 games behind the Mets in the National League East with 33 games to play rather than looking them square in the eye down the stretch. Point plenty of fingers. Start with them.

Last year’s top five Nats starters allowed 3.22 runs (all runs, not just “earned runs”) per nine innings. This year, the top five, the supposedly super quintet, have allowed 4.12 runs per nine frames. The math isn’t tough. At this rate, the five top starters will give up 75 more runs than last year. Subtract about eight wins. That’s crushing.

The Nationals have gotten back on track, but is it enough to overtake the Mets and win the division? (Post Sports Live/The Washington Post)

This was supposed to be a Nationals rotation to tell your grandkids about. Instead, the family dog now puts its paws over its eyes.

On Sunday, Strasburg, who had been the Nats’ hottest pitcher, left after four innings with a “little ball” in his back that causes spreading tightness and discomfort — the same problem that contributed to one of his two trips to the disabled list this season. The Nationals beat Miami anyway, 7-4, coming from behind, helped by two-run homers by Jayson Werth and pinch hitter Clint Robinson. But Strasburg’s dubious availability — short term, medium term — overhung the day.

“Grind through it, get through the year, then find out what it is,” Strasburg said. “It’s not muscular. They can’t really figure out what it is or what’s wrong.”

That might describe almost every aspect of the Nationals’ rotation this season.

The Nats, hard to believe, are actually on pace to score five more runs and hit 21 more homers than they did last season when they won 96 games. They’re also on pace to make five fewer errors than in 2014. The bullpen’s done some damage in the seventh and eighth, but the Nats are 55-0 when leading after eight innings.

It’s the mag-cover rotation that’s sinking this nine the most. One pitcher gets hot, but another gets hurt. One comes back, but another misplaces his slider or the sink on his fastball. Last year the starters were The Best by a lot. Now they’re barely top 10 in the majors. They aren’t bad. But for a team constructed around them, they aren’t special.

Rookie Joe Ross has been steady and calm, the best of the bunch in recent months. It makes you wonder, if a 22-year-old can do it, what’s their problem?

Whether some semblance of an elite rotation can be patched together down the stretch will probably be the key to whether the Nats can make one last run at the Mets. Like everything about this traumatic season, it will be a battle.

Ross is nearing his innings limit for 2015. Two or three more starts? Scherzer — cost $210 million — has a 5.01 ERA in his last 10 starts and has fallen to 11-11 as his team loses his matchups with one little-known rookie after another.

Fister, who got Sunday’s win in relief, has shown signs of improvement after being exiled to the bullpen for ineffectiveness (5-7, 4.56 ERA). “I’m back to normal,” he vowed Sunday. We’ll see. Vintage Fister would be a boost.

Roark (4.54 ERA) has been in the minors, stretching out his arm for starting duty in September when Ross is shut down. He has been jerked around so much it’s a wonder his arm still works. Gonzalez and Zimmermann have been decent but not up to past standards.

The key link is Scherzer, who was throwing no-hitters or near no-hitters in the spring. He has lost the usual sink and swerve (away from lefties) on his 95-to-99-mph fastball. That’s the pitch that separates him and sets up all his other pitches. Where has it gone? Who knows? Not Scherzer. Maybe to the Home For Wayward Mechanics along with Zimmermann’s slider and Fister’s command.

You want one answer to whether there will be a pennant race in D.C. this year? Scherzer has seven more starts. The rotation was reset so he faces the Mets twice. Win six of those games, including both against Mets’ horses. Then you get fun.

Everyone knows about the injuries. On Friday, Denard Span was lost for the year to hip surgery. Subtract 101 games in 2015 for the Nats’ 2014 NL hit leader. “I’ve never seen anything like it,” Manager Matt Williams of the waves of injury.

They have lost 80 games from Anthony Rendon, their best player in 2014, as well as 71 from Werth and 41 from Ryan Zimmerman. Yet the Nats are actually on pace to score 691 runs against 686 last year thanks to Bryce Harper’s monster season and the unexpected offensive contributions of Michael A. Taylor, Danny Espinosa and Robinson, all of whom were keys in Sunday’s win.

Taylor, who ran full-speed, face-first into the center field fence on Thursday, returned, the swelling in his right knee smaller but not gone. He had two hits, scored two runs and made a brave sprinting catch, saving a run for Fister before banging into the right field scoreboard.

“Any time a guy plays chicken with the wall,” said Fister, shaking his head appreciatively. “That thing’s not moving.”

For the second straight day, Robinson hit a vital two-run bomb. After taking a 3-0 pitch for a strike, he said he “got a good old-fashioned 99-mph heater on 3-1, looking for it. Upper deck, like it.”

There would be a lot to like about the still-fighting Nats, even without Span, if the rotation could get on a roll. Except for Strasburg, nobody even has a hangnail now.

Will they step up? Or will 2015 be remembered as a group photo of five proven star pitchers, all in Washington uniforms: one a Cy Young award winner, one a past 21-game winner. Another a 19-game winner. One the reigning NL strikeout champ and yet another with a 2.41 ERA last year. They make quite a portrait.

Will they let it end up with a black border around it?