It is the second time the highly touted 2009 top draft pick, who only made his major league debut in June, has been placed on the disabled list this season.
Strasburg left the game against the Philadelphia Phillies on Saturday during the fifth inning. It was just his third start since returning to action following a stiff right shoulder.
The 22-year-old had an MRI scan on Sunday and will undergo a second procedure to give the team a better look at the injury.
"It's a different type of MRI. The results, I think, will be back within a 24-hour period," Nationals general manager Mike Rizzo told the team's website (Washington.nationals.mlb.com).
"Stephen is anxious to find out what's going on."
Strasburg has a 5-3 record with a 2.91 ERA in 12 starts.
(Reporting by Jahmal Corner in Los Angeles; Editing by Greg Stutchbury)