Casey Janssen. (John Bazemore/AP)

JUPITER, Fla. — It’s not usual for Casey Janssen to feel some soreness in his shoulder as his workload builds and he prepares for the season. It normally goes away. But over the past week, the right-handed reliever’s shoulder still felt sore and didn’t rebound as usual. “It felt like it wasn’t going away as fast as it probably should have so that’s when I spoke up,” he said.

Janssen underwent an MRI on Sunday, not Monday as originally suggested by the Nationals. The results showed rotator cuff tendonitis, which Janssen has dealt with in the past. General Manager Mike Rizzo said he didn’t consider Janssen’s injury serious. Manager Matt Williams said Janssen is questionable for opening day.

“He knows how to take care of himself and get ready for the season,” Rizzo said. “He’s had this before. He knows how to battle through it.”

Janssen, who has dealt with shoulder soreness and had surgery in the past, said he isn’t sure what will happen next other than rest and treatment, nor is he sure how long he will need to do this. The veteran reliever who signed a one-year, $3.5-million deal in the offseason was expected to fill the set-up role left vacant by Tyler Clippard.

“I’m not concerned,” he said. “I hate missing any time. I hate missing any game, be it a spring training game or a real game. I feel like I work really hard to prepare myself. From that standpoint, it’s frustrating and you think you’ve crossed every T and dotted every I before you got here and while you’re here. When something like this happens, something didn’t go the way it’s supposed to.

“But I don’t see this being a major concern or event. Hopefully get back to throwing here soon and being a one-inning reliever things can move more quickly because it’s just an inning and I’ve put a nice foundation of workload up until this point. Hopefully you don’t lose it and once you feel good you take the wheels off and you go again.”

Janssen, 33, hasn’t pitched in a spring training game since March 23. He stopped throwing recently to rest. Manager Matt Williams is expected to address the test results and Janssen’s prognosis after Monday’s game.

“Once this thing says go, I’m going,” Janssen said. “Obviously the trainers are going to form a game plan but all that’s going to be based on is my comfort or discomfort level. It’s something that I know few arms feel amazing even at this stage of the game. I might not have to feel a million percent as long as you can manage it and get over that hump. Once you’re in that season, things flow a bit better.”