PHOENIX — Stephen Strasburg didn’t have it, and so the Washington Nationals didn’t either, and the result was what happens when a starter falls all the way flat.

The Nationals were crushed by the Arizona Diamondbacks at Chase Field on Saturday, 18-7, because Strasburg gave up nine earned runs in 4 ⅔ innings. That tied a career high for the 30-year-old, who earned National League pitcher of the month honors for July, and who was bullied from the start by a mediocre lineup that he can’t solve.

Strasburg allowed nine hits, walked two and gave up three home runs. The Diamondbacks scored two in the first, one in the second, two in the third and five in the fifth, polishing off Strasburg’s line when Alex Avila blasted a two-run shot off Matt Grace.

Reliever Tanner Rainey later struggled through the seventh, walking four to yield a run, firing two wild pitches and exiting with the bases loaded. Daniel Hudson replaced him and stranded three runners with a strikeout in his Washington debut. Then Gerardo Parra took the mound, becoming the first Nationals position player to pitch this season, and pumped 92-mph fastballs while giving up four runs. Then second baseman Brian Dozier relieved him, becoming the second Nationals position player to pitch this season, and gave up a moonshot home run to Eduardo Escobar.

But what mattered most, by night’s end, was Strasburg having his worst outing of the season against a sub-.500 team.That happens in baseball, an unpredictable game, known for bucking probability and logic whenever it pleases. But it was also a setback Washington cannot afford beyond this contest. They are still without Max Scherzer because of lingering shoulder issues. The back of their rotation is still a relative question mark. And so they need, maybe more than anything, for Strasburg to be an unshakable ace.

“It’s funny how this game is, there are some teams that pitchers own, and some teams come in and the team owns the pitcher,” said Manager Dave Martinez of Strasburg and the Diamondbacks, who now have seven homers off him this season. “You got, right now, our No. 1 guy out there who has pitched really well. They just hit the ball today.”

Strasburg had not allowed a home run in six starts, going back to June 21, when Austin Riley took him deep for the Atlanta Braves. He posted a 1.14 ERA in five July starts. His last losing decision came against the Diamondbacks in mid-June. They scored six runs on nine hits and knocked him out after five innings. Since then, he had dominated, giving Washington a chance every fifth day, softening Scherzer’s ongoing absence — if anything could.

Then Arizona got to Strasburg again.

“I don’t know what they were seeing . . . well actually I do know what they were seeing,” Strasburg said, though he didn’t expand on what the Diamondbacks may have picked up on him this year. “So that’s a positive. Games like this, when you get knocked around, you always try to look at positives and ways to improve. And I think I figured that one out.”

First came a two-run homer from Escobar in the bottom of the first, erasing the lead Washington grabbed in the top half. Strasburg saw Escobar take an uppercut swing, stalking his low-and-in fastball, then watched the ball arc into the right field seats. Next was Nick Ahmed’s turn, on a low sinker in the second, and Victor Robles tracked that drive until he ran out of room on the warning track. That homer was to straightaway center, landing above the 407-foot sign, leaving Strasburg with narrowed eyes and a shaking head.

The Diamondbacks added two more in the third, once David Peralta doubled in Ketel Marte, and Christian Walker singled in Peralta. Both hits came with two outs. Peralta crushed a curveball, while Walker lined a sinker that was off the plate inside. They gave the Diamondbacks a three-run lead, before Strasburg had recorded his ninth out, and the Nationals gave themselves a chance by scoring two in the fourth.

But Strasburg was instead knocked around some more, starting with a leadoff triple for Marte, then an RBI single for Escobar, then Jake Lamb’s two-run homer to left. That came off a first-pitch curveball that caught the outside edge of the zone. All six of the Diamondbacks’ run-scoring hits off Strasburg came within three pitches of the at-bat. And it didn’t matter where Strasburg threw the ball in those counts — down, in, away or off the plate — because he never found a rhythm in 98 pitches.

“They’ve tattooed me twice this year, that seems to be obvious,” Strasburg said. “But again, it’s my job to mix up pitches, execute pitches, and it’s their job to put the barrel on the ball and they did that tonight. Can’t really be too frustrated about it, but I do know I have to go out there and be better next time.”

He lasted one more batter after Lamb’s homer. He struck out Ahmed, seeming to end the inning, but his change-up struck the dirt and bounced away from Kurt Suzuki. Ahmed raced into first. Strasburg pointed to the ball before letting his long arms droop below his waist.

It was just that kind of night.