This is not the first time Janssen, 33, has dealt with shoulder issues. A torn right labrum caused him to miss the 2008 season. In November 2012, Janssen had surgery to repair lingering joint soreness in his right shoulder.
The Nationals said they are not concerned about the results of the exam because Janssen has experienced this soreness before and knows how to handle it.
"He knows how to take care of himself and get ready for the season," general manager Mike Rizzo said. "He's had this before so he knows how to battle through it."
Janssen will be shut down for the next few days, and his status for Opening Day is uncertain.
"That doesn't mean he won't be ready," manager Matt Williams said. "But the fact that he has not been throwing for a few days, that tells us with this amount of time left he would be a question mark."
Janssen signed a one-year deal with the Nationals during the offseason to become the team's setup man, replacing Tyler Clippard, who was traded to the Athletics for second baseman Yunel Escobar.
Janssen last pitched on March 23 against the Yankees and allowed two runs in one inning. It was during that game when he felt uncomfortable. Janssen was scheduled to pitch Saturday against the Mets, but he was scratched because he had a tough time getting loose.
"I'm trying to get ahead of it where we are not doing anything that would be a long-term thing," Janssen said. "… I don't see this being a huge thing. I didn't pitch in the last couple of days. It's never good, but I dealt with a lot worse."
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. Jamal Collier is a reporter for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.