Coffey, who is expected to return April 24, was 1-0 with a 6.32 ERA in five appearances this season. He was signed as free agent in the offseason and last pitched on April 8.
"It felt it like a cramp," Coffey said. "The doc looked at it a couple of times, everything checked out. It's just a small strain. It's a day-to-day type of thing. I'll be ready to go by [next] Sunday. No player ever wants to go on the DL, but it makes more sense this early in the season. You don't want to go out there and have something that is nagging. I'll take the seven days and let it heal."
With Coffey sidelined, right-hander Collin Balester was recalled from Triple-A Syracuse after working five scoreless innings of relief this season. Balester, 24, made 17 appearances for the Nationals in 2010, finishing with a 2.57 ERA and 12 strikeouts per 9 innings. Overall, he is 4-12 with 5.29 ERA in 39 career games, spanning three seasons at the Major League level.
Spencer Fordin is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.