The Nationals placed right-hander Jonathan Papelbon on the 15-day disabled list Tuesday, retroactive to Monday, with a right intercostal strain.
Right-handed reliever Matt Belisle was activated from the DL to replace Papelbon.
Papelbon, who has gone 16-for-18 in save opportunities this season, is making the first DL stint of his 12-year career. He ranks 10th all-time with 365 career saves.
Belisle returns to the Nationals after missing 45 games with a right calf strain. He had a 1.50 ERA in seven games with Washington before the injury, and he posted a 4.