Left-hander Gio Gonzalez heads to the mound Saturday, when he endured a second consecutive rough start. (Alex Brandon/Associated Press)

Steps from the exit to the Washington Nationals’ clubhouse, ready to leave behind a day that presented challenges unwelcome for any pitcher and unprecedented for him, Gio Gonzalez stopped. Head athletic trainer Lee Kuntz called his name and asked whether he could come to his office. Gonzalez nodded and walked with Kuntz to the other side of the room, passing through another door.

Gonzalez could handle one challenge that arose after he allowed the New York Mets five runs in three stilted, laborious innings during a 5-2 loss Saturday afternoon at sold-out Nationals Park, his shortest start as a National. Add in his implosion in Oakland last weekend, and never in his career had Gonzalez endured two starts in such brutal succession. So consistent for so long, Gonzalez now needs a rare rebound.

“That’s the beauty thing about baseball,” Gonzalez said. “You’re always going to have to learn something new. You’re going to have to figure it out.”

But Gonzalez confronted another, darker challenge, one he may not be able to conquer with plain resolve. He seems unsure whether his consecutive clunkers, the likes of which he has never endured in Washington, can be attributed to the need for adjustments — an inconsistent arm slot, say, or a preponderance of high fastballs — or poor health.

The Nationals plan for Gonzalez to undergo precautionary tests on his arm, including an MRI exam, on Sunday, according to a person familiar with the situation.

“You know, it’s one of those things, you just got to keep grinding through,” Gonzalez said when asked about his health. “Just trying to find out what it is. Hopefully, something positive comes out of it. Other than that, just try to stay positive.”

Asked specifically whether he had any health issues, Gonzalez paused and let out a long breath.

“Realistically, arm was dropping a lot,” he said. “I guess we’ll see.”

Had Gonzalez’s 84-pitch slog happened in a vacuum, it could be easily dismissed as a blip for a dominant pitcher. Because it happened six days after Gonzalez allowed seven runs over 41 / 3 innings in Oakland, the start caused a measure of alarm. The last time Gonzalez failed to pitch five innings in consecutive starts? April 2010, the first month of his second full season.

“I can’t put a finger on [Gonzalez’s health], because I don’t know,” Manager Matt Williams said. “There’s no difference in velocity. He’s running it in there at 94 miles per hour. It doesn’t feel like that. He’s had a little tight shoulder earlier this year but has continued to pitch and tells us that he feels good. So I don’t know. I don’t see that, though.”

On April 23, Gonzalez exited after five innings and only 83 pitches because of stiffness in his shoulder. Williams said the Nationals have continued to monitor Gonzalez’s shoulder as the left-hander has made every successive start on schedule. And he says he feels fine, Williams said.

Last weekend, Gonzalez’s fastball averaged 91.8 mph. He had averaged a slower fastball in only three starts since he arrived in Washington, two coming last April and another last July. But his average velocity surged back to 92.3 on Saturday. He hit 94 several times and topped out at 95.

“Just trying to stay loose most of the time,” Gonzalez said. “It was definitely tough. Other than that, arm slot wasn’t where I wanted it to be. It kept dropping. Fastball would come in hard and sometimes drop a little.”

For now, the Nationals can only hold their breath and mourn a loss that kept them a half-game out of first place.

Ian Desmond’s two-run homer in the fourth inning gave them a pulse, and Craig Stammen’s four scoreless innings kept them within striking distance. But Bartolo Colon held them down, and the Nationals never recovered after Gonzalez dug Washington’s latest first-inning hole.

Gonzalez’s curveball lacked its typical snap and electricity, and he hung fastballs off the plate to his arm side. He still induced 11 swinging strikes, but he also fell behind in counts constantly and left fastballs high in the strike zone.

“I’m falling behind on everyone,” Gonzalez said. “It definitely will pay a toll on your pitch count. It slows the game down a lot, gets the players on their heels. I didn’t give any spark to the game, especially for the guys. They were trying their best to look at the bright side. It was tough. I didn’t give them any chance to come back and find some energy.”

The Mets pounced from the start. Gonzalez had allowed three runs in the first frame for the second time in a week, instant ineffectiveness that prolonged a trend that’s moved from baffling to troubling. In their first 42 games, Nationals starters have yielded at least three runs 10 times. The other 29 big league teams have done it 58 times, an average of just two apiece.

In the third inning, with one out and a runner on first, Gonzalez fired a 92-mph fastball down the middle of the plate. Juan Lagares blasted it into the red seats, sending the Mets ahead 5-0 and finalizing the damage against Gonzalez. His ERA rose to 4.62, up from 2.91 one week ago.

“Inconsistent,” Williams said. “Way too many pitches, missing the zone, not throwing it where he wanted to. Just inconsistent. And getting behind. If you get behind at this level, you’re going to pay for it.”

Gonzalez finished his rotten afternoon with one final indignity. After Nate McLouth doubled to lead off the third inning, Gonzalez walked to the plate as Stammen warmed up in the bullpen. Williams chose to save his pinch hitters for later innings, instead asking Gonzalez to bunt McLouth to third with one out.

“We don’t know how that game is going to go,” Williams said.

Colon hurled an 88-mph fastball, and Gonzalez popped it up between the mound and the plate. He started walking back to the dugout before it even nestled into Colon’s glove. A portion of the sellout crowd booed. After groundballs from Denard Span and Anthony Rendon, McLouth never advanced past second.

Stammen dominated for four innings, allowing one hit and striking out two, requiring only 54 pitches for the longest relief appearance of his career. “There’s not a lot of long relievers that go down in history, but he’s well on his way,” Desmond said. “This guy is unbelievable.”

Stammen saved the bullpen for Sunday’s game, but he could not save the Nationals. Nothing could ease the worry over Gonzalez, which will persist until — and if — he gets a chance to recapture his usual form five days from now.

“It’s part of baseball,” Gonzalez said. “You’re always going to learn something new. Something new is going to happen. Hopefully, I’ll figure it out.”