Mattheus had an X-ray and was expected to visit the Nationals' foot specialist in Baltimore on Tuesday. Manager Davey Johnson said he hoped the right-hander would be back for Tuesday night's game.
"He thinks something might have broken up in there that's been bothering him for two weeks," the manager said. "He came in and iced it, and we're treating that just like anybody who's got something wrong. Look at it 10 ways from Sunday."
Right-hander Chien-Ming Wang, who is on the disabled list with a strained left hamstring, is scheduled to throw on the side at Citizens Bank Park on Tuesday. It's possible that, if Mathheus had to go on the DL, Wang could be activated to take his spot on the roster.