Strasburg grabbed his right wrist and shook it after a pitch to Dominic Brown on Saturday night. Manager Jim Riggleman immediately left the dugout to check on his ace.
After a brief conversation, Strasburg left. In 4 1/3 innings, he gave up two hits, one run and struck out six as the Nationals rolled to an 8-1 win.
Nationals general manager Mike Rizzo said the young pitcher would have an MRI exam Sunday.
"You're always concerned when a pitcher leaves. We'll see what the MRI says," Rizzo said.
Strasburg was making his third start since returning from a stint on the DL with inflammation in the back of his right shoulder.
Craig Stammen relieved Strasburg with the Nationals leading the Phillies 5-1.
Information from The Associated Press was used in this report.