After throwing a 1-1 pitch to Domonic Brown for a ball with one out in the fifth, Strasburg was grimacing in pain and shaking his arm.
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 left the game 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.
"The thing was he pounded the strike zone," said catcher Ivan Rodriguez. "He got ahead, used the fastball a lot. The breaking ball was very good today. He struck out a few guys with the changeup. That tells you how good he is. He has three pitches to strike a lot of guys out. He did it tonight."
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."
Doug Slaten ended up the winning pitcher, tossing 1 2/3 scoreless innings after Stammen. It helped that the Nationals scored early off right-hander Kyle Kendrick.
Washington scored three runs in the first inning. Rodriguez capped the frame with a two-run single.
Two innings later, Ryan Zimmerman scored on an Adam Kennedy single, while Roger Bernadina scored on an RBI groundout by Strasburg.
The Phillies scored their lone run on an RBI groundout by Ryan Howard, who returned from the disabled list, in the fourth.
"I thought it was pretty good," Howard said about Strasburg. "It was an easy 98, 99 mph on his fastball, pretty good changeup. It had some sink to it. He had some pretty decent stuff.
"You never want to see anybody get injured. 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."
Bernadina ripped a three-run homer to pad the Nats' lead, then scaled the wall to rob Howard of an extra-base hit in the ninth.
"I didn't know if the ball was gone. I did whatever I needed to do to keep the ball in the ballpark," Bernadina said. "I'll go after anything."