(Photo by Jonathan Newton / The Washington Post)

Reliever Ryan Mattheus will undergo an MRI this afternoon to determine the severity of a chest injury that has momentarily paused his effort win back a spot in the Nationals’ bullpen.

Nationals doctors have told Mattheus they believe he has irritation in a joint on the inside of his right pectoral muscle. Mattheus first felt pain in his chest after he threw his second bullpen session of the season, and the Nationals promptly shut him down from throwing.

“It’s just kind of a bizarre thing,” Mattheus said.  “I don’t feel it really just walking around. But if I take big deep breaths, or cough or sneeze, or anything like that, it grabs me. It’s just in a weird spot.”

Mattheus will have a better idea of his timetable to return after the Nationals review his MRI results. As Mattheus understands it, he could miss between one and two weeks before he can throw again. In general, whatever time a pitcher spends not throwing, he must take the same amount time to rebuild his arm strength.

“They don’t think it’s anything real, real serious that’s not going to go away or linger for a couple months,” Mattheus said. “That’s the good part. But we don’t know exactly.”

If Mattheus needs to rest for two weeks – which would hold him out of competition for roughly four weeks – it would hurt his chances to stand out in a crowded competition for the final spots in the Nationals’ bullpen.

Mattheus posted a 6.37 ERA last season and missed almost two months after he broke his hand punching a locker. But he was also a key member of the Nationals’ 2012 NL East title team. He entered spring feeling strong and eager to reassert himself.

“It is real frustrating,” Mattheus said. “I was feeling really good, in good shape. I felt like I was in a good spot. Hopefully, this is just a minor setback.”

** No live batting practice today for the Nationals. Pitchers will compete in some kind of hitting game inside Space Coast Stadium. In every player’s locker this morning, there was a chart entitled, “Situational Play Matrix — Pop Up Priority.” It detailed every players’ job for every location of infield fly ball under every situation. Humorously, Wilson Ramos’s job in every circumstance was, “Cover plate.”