On top of that, manager Davey Johnson said that right fielder Jayson Werth is still a couple of weeks from coming off the disabled list after another MRI on his strained right hamstring showed he isn't healing as quickly as hoped.
With all that going on, an 8-0 loss to the San Francisco Giants seemed like an afterthought.
Mattheus allowed five runs in one inning of Washington's 13-4 loss to the San Diego Padres. According to Johnson, Mattheus did not inform the team of his injury until just before Monday's game at AT&T Park.
"Last night after the game he came in and was in the locker room and banged his glove against his locker with his hand in it," Johnson said. "He didn't say anything to anybody. His hand didn't swell up but when he went out to throw today his hand swelled up and he couldn't throw the ball."
The news came on the same night that the Giants lost starter Ryan Vogelsong to a fractured right hand while batting.
Mattheus underwent a series of tests Monday, and an X-ray revealed the damage.
Washington pitcher Zach Duke, who lasted 3 1/3 innings and took the loss against San Francisco, suffered a similar injury in 2011 when he was hit in his pitching hand by a line drive.
"When it's the throwing hand it's tough," Duke said. "You basically lose all the stamina that you build up in the arm. I couldn't pick up a ball for five weeks. You have to get out there and re-establish what you did in the offseason."
The Nationals are expected to call up two pitchers from the minors to help their beleaguered bullpen.
Reliever Henry Rodriguez pitched two innings in relief of Duke and threw 47 pitches, one day after throwing 21 in an outing against the Padres.
"It'll be tough to grind through without [Mattheus] for a little while, but he'll be back and I'm sure he'll be eager to go," Duke said.
Harper went 0-for-4 in his return to the Nationals' lineup after last season's NL Rookie of the Year missed two games with a bruised left knee. He was hurt in a hard collision with the outfield wall a week earlier at Dodger Stadium that also caused him to need 11 stitches on his chin.
Duke (0-1) lasted just 3 1/3 innings in a spot start for Ross Detwiler, who is sidelined with back spasms. Duke allowed four runs and seven hits in his first start of the year.