Mattheus broke his right (pitching) hand after a rough appearance against the Padres on Sunday when he punched a locker in frustration. He told reporters he wasn't aware the hand was broken until the next day, when the hand started swelling after Mattheus played catch.
The 29-year-old reliever then alerted pitching coach Steve McCatty, general manager Mike Rizzo and manager Davey Johnson, and the team placed him on the disabled list.
Mattheus left the Nationals' West Coast road trip early and flew back east to see a hand specialist, who determined that the broken bone would not require the insertion of a stabilizing pin, according to the source. The bone will be able to heal on its own, but will obviously need time to do so.
Mattheus' right hand will be immobilized and he will not be able to pick up a ball and start throwing for somewhere around the next month.
Depending on how quickly the bone heals and how long it takes Mattheus to build up arm strength lost during this time off, he could miss anywhere from 4-8 weeks, but six weeks is viewed as the more realistic timetable. That would have Mattheus returning in July, when he could be ready for the Nationals' stretch run.
Mattheus has been one of the Nats' key setup guys over the last two seasons. He had posted a 2.35 ERA through 13 games prior to his appearance against the Padres on Sunday, when he allowed five runs in just an inning of work.
For the season, Mattheus has a 4.96 ERA with 12 strikeouts and five walks in 16 1/3 innings.
Left-hander Fernando Abad, called up from Triple-A Syracuse after Mattheus' injury, will likely get some of the work Mattheus would have received, serving as a bridge from the starters to the late-inning relievers.