With the Nationals leading, 5-1, with one out in the fifth, Strasburg was facing Domonic Brown. On a 1-1 pitch, Strasburg threw a ball, but was grimacing in pain after the pitch and shaking his right arm.
"It's [too bad] that he got hurt. [Home-plate umpire] Brian [O'Nora] said he heard something pop," Phillies center fielder Shane Victorino said.
Strasburg then motioned the trainer to come to the mound. Manager Jim Riggleman, pitching coach Steve McCatty and head athletic trainer Lee Kuntz came to see what was wrong with Strasburg, who wanted to stay in the game, but left a few minutes later.
Riggleman said he would have kept a veteran like Livan Hernandez in the game if he had asked to stay in, but not a rookie like Strasburg.
"[Strasburg] said, 'I feel good. I don't even feel anything. I want to keep pitching.' We choose not to do that," Riggleman said.
Strasburg threw 4 1/3 innings, allowing one run on two hits, striking out six batters and walking none. He threw 56 pitches, including 38 strikes.
"He was throwing the ball very well," Riggleman said. "You hate to see anybody show some signs that they are a little tender out there. Certainly with Stephen, we are going to be a little careful. We just hope for the best. He's getting looked at tomorrow, and hopefully we'll get good news."
Right-hander Craig Stammen entered the game to finish off the inning. Stammen saw what was going on with Strasburg and figured that he would have to get in the game right away.
"I was the only one in the bullpen who actually saw him shake his arm," Stammen said. "And I looked in the dugout. I said, 'Oh, that might be me [coming into the game].' I didn't know if I was going to throw in the bullpen or go straight to the mound. I went straight to the mound. That's what I did.
"The only question was, how many pitches I was going to throw. It was kind of awkward, because everyone was waiting for the game to start and I was trying to get loose."
The rival Phillies wished Strasburg a quick recovery.
"You never want to see anybody get injured," Ryan Howard said. "I don't know exactly what it was -- it looked like he had some sort of discomfort in his arm after he threw the pitch. I hope he has a speedy recovery."
Kyle Kendrick, who started opposite Strasburg, wished the phenom the best as well.
"Scary," Kendrick said. "I guess they said they heard his elbow pop. That's never good, hopefully everything turns out well."
And during his short outing, Strasburg left a strong impression on Phillies manager Charlie Manuel.
"I didn't want to see him leave that way," Manuel said. "He's got good stuff. He really has got tremendous stuff."
It marks the second time Strasburg had to be shut down because of an injury. On July 28, Strasburg was scratched from his start against the Braves after experiencing shoulder stiffness during his warmups in the bullpen.
Instead, reliever Miguel Batista made the spot start for Washington. And he delivered with a brilliant effort, throwing five scoreless innings in his first start of the season as the Nationals shut out Braves, 3-0.
Strasburg, the No 1. overall pick in the 2009 First-Year Player Draft, was warming up just minutes before the game when McCatty and Kuntz noticed that something was wrong with the righty after eight pitches. Strasburg told McCatty that he was stiff but could pitch in the game.
Riggleman and general manager Mike Rizzo were informed about Strasburg's problem, with Rizzo pulling the plug on Strasburg's start. Strasburg was placed on the disabled list the next day.