(Bill Streicher-USA TODAY Sports)

All-star right-hander Jordan Zimmermann, the starting pitcher the Nationals view as their ace, exited in the fourth inning of his start Friday night with a cramp in his right biceps. Zimmermann will undergo an MRI Saturday to determine the severity of the injury, but both he and Manager Matt Williams expressed optimism.

“I don’t think it’s anything major, to be honest with you,” Zimmermann said.

Williams said the cramp was located in the “belly” of Zimmermann’s biceps, which is considered a positive sign. Sometimes, a serious elbow injury will manifest as biceps pain. The Nationals did not initially believed that is the case. But “that’s why we’re getting the MRI,” Williams said.

Zimmermann will not pitch in the All-Star Game for the second straight season. Last year, a stiff neck knocked him out of the showcase after he earned his first selection.

Zimmermann threw Phillies catcher Cameron Rupp a 1-2, 88-mph slider, his 53rd pitch of the night. He wiggled the fingers on his right hand and waved to the Nationals’ dugout with his glove. Nothing seemed different about his delivery, but for starting in the third inning his biceps muscle had started to feel more and more tight.

Manager Matt Williams, pitching coach Steve McCatty and head trainer Lee Kuntz went to the mound. They spoke with Zimmermann for about 30 seconds. McCatty turned and yelled at home plate umpire, “Stammen” – they knew Zimmermann would need to leave the game, and reliever Craig Stammen would take his place. Stammen stirred. Zimmermann walked off the mound and stared into the middle distance. Everyone associated with the Nationals sees Zimmermann as a rugged competitor, a pitcher who not leave a start barring extreme circumstance.

“I feel okay now,” Zimmermann said. “It felt a little tight that last inning. Every pitch, it felt like it was getting tighter and tighter, like it was cramping up. I didn’t want to push it too far and have something worse happen.”

A serious injury to Zimmermann, who underwent Tommy John surgery in 2009, would be one of the most devastating blows the Nationals could suffer. Sunday afternoon, he was named to his second consecutive All-Star Game. He entered Friday night at 6-4 with a 2.79 ERA, having allowed seven runs since the start of June.

In the first three innings Friday night, he yielded four. Zimmermann 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 the Phillies hammered three consecutive 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 threw Brown, the hitter who led off the fourth, two 94-mph fastballs en route to a pop-up. He would face one more batter and throw four more pitches, and then the Nationals would start to hope.