Washington, DC (SportsNetwork.com) - Washington Nationals reliever Craig Stammen has a torn flexor in his right forearm and will require surgery, manager Matt Williams announced Thursday.

There is no date set for the surgery, and the timetable for his recovery will be decided on the extent of the tear.

The Nationals placed Stammen on the 15-day disabled list Wednesday with stiffness in his right forearm.

The 31-year-old has appeared in five games this season, allowing two hits, walking three and striking out three over four innings.

To bolster the bullpen, lefty Felipe Rivero was called up from Triple-A Syracuse.