The Nationals officially made the move Tuesday. Right-hander Jordan Zimmermann will take Strasburg's place in the rotation starting on Thursday.
To replace Strasburg on the active roster, the Nationals recalled right-hander Collin Balester from Triple-A Syracuse.
Strasburg hurt his forearm on Saturday in an 8-1 Nationals victory over the Phillies. With the Nats leading, 5-1, with one out in the fifth, Strasburg was facing Phillies outfielder Domonic Brown when -- on a 1-1 pitch -- he grimaced in pain and shook his right arm.
Strasburg then motioned for the Nationals' trainer to come to the mound. Riggleman, pitching coach Steve McCatty and head athletic trainer Lee Kuntz came out to see what was wrong with Strasburg, who wanted to stay in the game but left a few moments later.
In that outing, Strasburg pitched 4 1/3 innings, allowing one run on two hits, striking out six batters and walking none. He threw 56 pitches, including 38 strikes.
On Sunday, Strasburg had an MRI on the forearm, and the team doctor didn't like what he saw, so Strasburg will have an arthrogram to get a better look at the arm.
"The arthrogram is going to be performed [in Washington]," general manager Mike Rizzo said. "It's a different type of MRI where they inject dye into the area. Right now, we see a flexor issue. To get a clear diagnosis, we've gone to the extreme of going to an arthrogram. The results, I think, will be back within a 24 hour period.
"Stephen is anxious to find out what's going on. He felt good today. His elbow felt good. He wants a resolution to find out what's going on. To say he's a little anxious and a little emotional would be fair to say. He said he had gone through it at [San Diego State] and pitched through it."
It marked the second time Strasburg had to be shut down because of an injury. On July 28, the right-hander 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 the Braves, 3-0.
Strasburg, the No. 1 overall pick in the 2009 First-Year Player Draft, was warming up just minutes before that 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 Rizzo were informed of Strasburg's problem, with Rizzo pulling the plug on Strasburg's start. Strasburg was placed on the DL the next day.
In 12 starts and 68 innings this season, Strasburg is 5-3 with a 2.91 ERA, having struck out 92 batters and walked 17.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. Jeff Seidel contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.