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The Nationals placed Willingham on the 15-day disabled list on Thursday. Manager Jim Riggleman said Willingham will have surgery next week to repair a medial meniscus tear in his left knee.
It is unlikely Willingham will have time to recover from the surgery before the end of the season, Riggleman said.
Willingham was hitting .268 with 16 homers and 56 RBIs.
Outfielder Nyjer Morgan was activated from the disabled list and batted leadoff in Thursday afternoon's 6-2 win over the Atlanta Braves. He had two hits and two runs in five at bats and also stole his 30th base of the season.
Thursday was the first day Morgan, who was recovering from a sore hip, could come off the disabled list.
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