Nationals starting pitcher Jordan Zimmermann, left, talks with team personnel during the fourth inning. Zimmermann left the game after the conference with what is being described as a biceps cramp and will have an MRI exam. (Matt Slocum/Associated Press)

Most everyone associated with the Washington Nationals views Jordan Zimmermann not only as their ace but also as perhaps their most rugged competitor, a stoic pitcher to whom begging out of a start is a foreign act. In college, he pitched with his jaw wired shut. As a rookie, he never left the outing in which he frayed an elbow ligament. On Friday night, then, a jarring scene unfolded at Citizens Bank Park. Zimmermann stood on the mound and summoned his dugout for rescue.

“I never want to call anybody out there,” Zimmermann said. “I knew my best interests were for me to come out of that game and get some work done and see what was really wrong.”

A cramp in his right biceps forced Zimmermann, the Nationals’ lone all-star and best starting pitcher this season, to leave in the fourth inning of the Nationals’ 6-2 loss to the Philadelphia Phillies. Zimmermann will undergo an MRI exam Saturday to determine the full nature and extent of the injury.

The cramp will prevent Zimmermann from pitching in the All-Star Game. But both he and Manager Matt Williams expressed optimism the cramp did not portend a more serious issue with his elbow or shoulder.

“I don’t think it’s anything major, to be honest with you,” Zimmermann said. “It’s a little cramping, and it just felt really tight. So I didn’t want to push it.”

Williams said the cramp was located in the “belly” of Zimmermann’s biceps, which is considered a positive sign. Zimmermann said his shoulder felt “fine” and his elbow felt “great.”

Sometimes a serious elbow injury to a pitcher will manifest as biceps pain. The Nationals believe Zimmermann, who underwent Tommy John surgery in 2009, dodged that issue. “Well,” Williams said, “that’s why we’re getting the MRI.”

In the fourth inning, Zimmermann threw Phillies catcher Cameron Rupp a 1-2, 88-mph slider. He wiggled the fingers on his right hand and waved to the Nationals’ dugout with his glove.

In the dugout, Williams’s pulse quickened. In left field, Bryce Harper studied to see how Zimmermann moved his right arm. Catcher Wilson Ramos thought, “This is bad.”

Nothing had seemed different about his delivery or his pitches. His fastball hummed at 94 mph, his sliders around 88. But something had gone awry.

“It was just getting a little tight the last inning, and every pitch it seemed like it was getting tighter and tighter and cramping up,” Zimmermann said. “I didn’t want to push it too far and have something worse happen.”

Williams, pitching coach Steve McCatty and head athletic trainer Lee Kuntz strode to the mound. Zimmermann placed his glove over his mouth and told the assemblage he felt a cramp in his biceps. McCatty turned and yelled at home plate umpire, “Stammen.” They knew Zimmermann needed to leave, and reliever Craig Stammen would take his place. Stammen stirred. Zimmermann walked off the mound and stared ahead.

“It’s definitely scary,” Harper said. “He’s had Tommy John before.”

A serious injury to Zimmermann, who has not gone on the disabled list since the Tommy John surgery, would be one of the most devastating blows the Nationals could suffer. On Sunday afternoon, he was named to his second consecutive All-Star Game. He won 19 games last year, and he entered Friday night 6-4 with a 2.79 ERA, having allowed seven runs since the start of June.

“If he can’t pitch for a while, it’s a big deal,” Stammen said. “He’s been our most consistent. He’s our all-star. We’ve got guys that hopefully would be able to come and up replace him and do just as good of a job.”

Zimmermann at least will miss the All-Star Game for the second straight year. Last season, he missed his first chance to pitch in the showcase because of a stiff neck. Cardinals Manager Mike Matheny will choose Zimmermann’s replacement. Williams planned to speak with Matheny on Saturday, but the replacement does not need to come from Washington, even though Zimmermann remains its lone representative.

“That’s not the main problem right now,” Zimmermann said. “It’s getting healthy. Hopefully there will be another one down the road.”

In the first three innings Friday night, Zimmermann yielded four runs. He breezed through the first inning on nine pitches. When he dusted Ryan Howard with a slider to start the second, it appeared he had brought the same top form from the past month with him to Citizens Bank Park.

And then three consecutive Phillies hammered base hits, culminating with Domonic Brown’s two-run double to right-center field. Jimmy Rollins demolished a two-run home run in the third inning. He trailed 4-0, but he also ended the third with two strikeouts, throwing a 94-mph fastball past Cody Asche.

“He was throwing good,” Ramos said.

Zimmermann threw Brown, the hitter who led off the fourth, two 94-mph fastballs en route to a popup. He would face one more batter and throw four more pitches, and then the Nationals’ hearts moved into their throats.

“I know he’s not going to come out of the game unless he’s probably hurting really bad,” Stammen said. “I guess that’s in the back of your head.”

The Nationals, who remained tied for first place in the National League East with the Atlanta Braves, fell into a 5-0 hole before the seventh inning, when Harper injected one flicker of light into an otherwise dreary night. He roped A.J. Burnett’s 1-0 fastball into the right field seats, snapping a 4-for-33 slump with his first homer since he came off the disabled list June 30.

“It felt really good,” Harper said. “The past couple days, I feel like my swing has been pretty good. I’m still trying to work every day, really just come in and do my routine, try to just have good A-Bs.”

Harper also had a chance to thrust the Nationals back into the game in the eighth. With two outs, Ryan Zimmerman drilled an RBI double to cut their deficit to 6-2, bringing Harper to the plate with two runners in scoring position. Harper had been aggressive at the plate all night, putting three of the four pitches he had seen into play. He made it four of five when he skied Ken Giles’s first-pitch, 100-mph fastball into foul territory to end the inning.

By the end of the night, Zimmermann’s cramp had calmed down. He still wondered what the immediate future, just like everyone else in the clubhouse.

“It calmed down a little bit,” Zimmermann said. “But if I had to go and play catch tomorrow, I don’t know how it would be. We’ll get some tests done and see what the deal is.”