"We'll see how it goes the next few days," Nationals manager Dave Martinez said. ''It's something you've got to be careful about."
Fedde is 4-4 with a 3.90 ERA in 11 starts. He had thrown 21 consecutive scoreless innings before giving up five runs in four innings in the Nationals' 13-12 win over Philadelphia on Wednesday.
Fedde first felt an issue with his side the day after the game and underwent an MRI, Martinez said.
"He had no idea how he got it," Martinez said.
Fedde had been scheduled to start Monday's game in Washington against the New York Mets. The Nationals have not yet named a replacement to make that start.
Washington recalled right-handed pitcher Andres Machado from Triple-A Rochester, where he had a 2.63 ERA with 18 strikeouts in 13⅔ innings as a reliever.
The Associated Press contributed to this report.