"It's an interesting spot," first-year manager Matt Williams explained at the time. "It's kind of chest, front, which is odd," he said. "So we're going to have to find out exactly what it is and then take the steps accordingly to get it taken care of, but at this point we just don't know."
Mattheus was eventually diagnosed with costochondritis, which the Washington Post's James Wagner explained, is, "... the inflammation of the joint between the joint that connects the sternum and ribs."
"It's where the rib head hits the cartilage and it's irritated," Williams said after they received the diagnosis. "So, what he's doing right now, he's doing all the conditioning stuff that they've got him on. Certainly trying to let it calm down. He'll continue to do that."
The right-hander didn't face hitters until mid-March, so he wasn't ready for the start of the season. As Opening Day approached, the Nationals' manager told reporters including MLB.com's Bill Ladson that Mattheus wouldn't be ready to pitch in the majors at the start of the season.
"'He has to try to build arm strength. To take him into a pressure situation right now with him not having enough innings in spring -- it's not fair,'" Williams said.
The Nationals optioned Mattheus to Triple-A Syracuse on March 25th. In nine games and 10 IP so far this season, he has a 5.40 ERA, a 3.95 FIP, three walks (2.70 BB/9) and eight Ks (7.20 K/9).
Washington Post writer Adam Kilgore reported tonight that Mattheus is on his way to Houston to rejoin the Nationals' bullpen when they start a two-game set with the Astros on Tuesday night. Citing "multiple people familiar with the situation," Kilgore wrote that though there's no word on which player Mattheus will replace on the roster, he will join the team.
In his breakout 2012 campaign, Mattheus made 66 appearances out of the pen for the NL East Division winners, posting a 2.85 ERA, a 4.42 FIP, 19 walks (2.58 BB/9) and 41 Ks (5.56 K/9) in 66 1/3 IP over which he was (5-3). Mattheus missed significant time in 2013 when he suffered a broken bone in his right hand while punching a locker after a particularly tough outing against the Padres in San Diego.