It's not yet known how Papelbon, who had never before been on the DL during his 12-year career, suffered the injury. But the 35-year-old wasn't available to pitch during last night's 4-1 win over the Cubs, watching from the dugout as Shawn Kelley instead recorded the save.
Manager Dusty Baker offered up a vague explanation of Papelbon's unavailability after the game, saying only that something other than his arm was "sore."
"We really don't know yet," Baker said at the time. "Paps was feeling pretty sore, and he was ailing, so we didn't really have Pap tonight. We'll make a further evaluation tomorrow."
Papelbon last pitched Sunday against the Phillies, giving up a homer to Maikel Franco in the top of the ninth but ultimately getting credited with a win after his teammates rallied from behind in the bottom of the inning.
In 25 appearances so far this season, Papelbon has a 3.28 ERA, converting 16 of 18 save opportunities. Since being acquired by the Nationals from the Phillies last July, he owns a 3.17 ERA in 47 games, with 23 saves in 27 opportunities.
With their closer sidelined for at least the next two weeks, the Nationals will have to figure out how to handle ninth-inning duties. Kelley has five career saves and is the most experienced member of the bullpen. Left-hander Felipe Rivero has electric stuff and has been viewed as a potential closer-in-waiting by some in the organization, but has struggled lately against left-handed batters.
Belisle rejoins the Nats bullpen after spending the last month and a half on the DL with a right calf strain. The 36-year-old reliever had been on an extended rehab assignment, totaling 11 innings in nine games with high Single-A Potomac, Double-A Harrisburg and Triple-A Syracuse, while the club waited to activate him until a spot in the big league bullpen opened.