But Strasburg said his shoulder was feeling much better than on Tuesday, and he is looking forward to coming back.
"They're taking every precaution with me," he said. "It's doing great already. [I'm] making big strides ... and when the time comes, I'm going to be ready to go."
Strasburg is not going to throw for the next three or four days, and his right shoulder will be re-evaluated by the team after that.
Nationals manager Jim Riggleman shrugged off the somewhat-disparaging remarks from Sen. Jim Bunning, a former Major League pitcher, about Strasburg's not starting on Tuesday night. Bunning said he always took the ball, no matter how he felt.
But Riggleman said because Strasburg is still very young, the team will be more cautious with him. If he were a veteran whose arm and shoulder often felt stiff and sore when warming up, that would be a different story.
"When [Bunning] was pitching at 30 years old, no question he's going to do that," Riggleman said. "When he was pitching, if he pitched at 21 or 22, they may not have let him pitch at that point, because you just don't have any history to go by. It's just a whole different dynamic when it's a young guy as opposed to a veteran guy."
Strasburg was scheduled to start Sunday's series finale against the Phillies. There's no official word yet on who will pitch.
One thing is certain, right-hander Jason Marquis will not pitch on three days' rest like Ross Detwiler did last Sunday against the Brewers. Marquis pitched Wednesday, allowing four runs in five innings for Triple-A Syracuse.
It also appears that right-hander Livan Hernandez will not pitch on short rest after throwing just 4 1/3 innings and allowing three runs on six hits Wednesday against the Braves.
Left-handers John Lannan and Matt Chico are candidates to make the start. Lannan is having an up-and-down season in the Minor Leagues. In seven starts for Double-A Harrisburg, he is 1-4 with a 4.20 ERA.
Chico is having a solid season in the Minor Leagues, going a combined 6-6 with a 3.46 ERA in 19 starts for Harrisburg and Syracuse.
The Nationals could also go back to the right-hander Batista, who pitched five shutout innings in Strasburg's place Tuesday. As of Thursday afternoon, Batista was not told whether he would start against the Phillies.
Before Thursday's announcement on Strasburg, Riggleman stressed the club's commitment to making sure the pitcher is feeling right.
The team has placed a limit on how much Strasburg will pitch this season. The total is scheduled to be 150-160 innings, including everything he did in the Minors before being called up.
"Our first priority is to take care of Stephen Strasburg for the future of this organization," Riggleman said.