The Nationals also sent outfielder Eury Perez to Triple-A Syracuse and recalled lefty Fernando Abad and right-hander Yunesky Maya to bolster their beleaguered bullpen. Mattheus' move is retroactive to Monday.
Mattheus allowed five runs in one inning of Washington's 13-4 loss to the San Diego Padres on Sunday. Manager Davey Johnson said Mattheus did not inform the team of his injury until just before Monday's 8-0 loss at San Francisco.
Mattheus will see a hand specialist in Baltimore on Wednesday to see if he will need surgery. He is expected to be out at least six weeks, Johnson said.
Calling up two pitchers left the Nationals with only four bench players.
"Hopefully the starting pitchers will hit three times and I won't need them much," Johnson said.
Abad has a 1.06 ERA with 12 strikeouts and two walks in 17 appearances for Syracuse this year, while Maya is 1-4 with a 5.07 ERA in eight starts.
With an off day Thursday, Washington will skip Ross Detwiler's spot in the rotation. Detwiler missed his start Monday because of back spasms.
Detwiler did some running and played catch before Tuesday's game. The Nationals are hopeful Detwiler will throw a bullpen session by Friday and be in line to start Tuesday, when his spot in the rotation would be due up again.