Strasburg flew to Washington to be examined on Wednesday after he complained of tightness in his neck and shoulder in Tuesday night's 11-6 win over the Braves. The 32-year-old was pulled after only 1⅓ innings.
Strasburg's injury adds more stress to an already thin Nationals rotation. Left-hander Jon Lester was to start Wednesday night's game on only three days' rest.
Manager Dave Martinez said he and his staff were "banging our heads now trying to figure out something for Sunday'' when Strasburg was scheduled to make his next start at Philadelphia.
Right-hander Erick Fedde, on the 10-day IL because of an illness, could soon rejoin the rotation. He is scheduled to make a rehab start for Class A Wilmington on Thursday. If that schedule is not changed, he would not be a candidate to start in Sunday's game.
This is Strasburg's second stint on the injured list this season. He was shut down from April 15 until May 21 with right shoulder inflammation. That recent injury contributed to the decision by Martinez to remove the right-hander so early from Tuesday night's game. Martinez said Strasburg "was a bit frustrated'' when the two spoke on Wednesday.
"He came back and felt good and had these issues with his neck going on yesterday,'' Martinez said. "I told him let's get him checked out and see where he's at.''
Strasburg left Tuesday night's game after William Contreras' liner hit his left arm near his glove in the second inning. Martinez and his medical staff were concerned about the pitcher from the start of the game, after he walked Ronald Acuna Jr. on four pitches.
Strasburg allowed one run on one hit and two walks while throwing only 30 pitches. He is 1-2 with a 4.57 ERA in five starts.
This will be the 14th visit to the injured list during Strasburg's career. He has won 113 games in 12 seasons, all with Washington, but has made only seven starts since posting an 18-6 record and 3.32 ERA in 2019.
"I know he's frustrated because he feels he let the team down and let me down, but I don't want him to even think like that,'' Martinez said.
The Nationals recalled right-hander Kyle McGowin from Triple-A Rochester. McGowin has a 3.09 ERA in 10 appearances, all in relief, for Washington this season.
Strasburg is in the second season of a seven-year, $245 million contract he signed after leading the Nationals to the World Series title in 2019, when he won MVP honors and went 5-0 with a 1.98 ERA in the postseason.
The Associated Press contributed to this report.