Manager Matt Williams said Tuesday that McLouth will have surgery Thursday for a torn labrum and is expected to recover in time for spring training.
Williams said McLouth has had discomfort in his throwing shoulder all season. The symptoms worsened after he dove for a ball in Miami in late July.
McLouth last played Aug. 1 and is on the 15-day disabled list. He is in his first season with the Nationals and in 79 games hit .173 with one home run and seven runs batted in.
The St. Louis Cardinals put second baseman Mark Ellis on the 15-day disabled list, retroactive to Aug. 15, because of a left oblique strain.
Ellis, 37, has not played since Thursday against the San Diego Padres.
David Price returns to Tampa Bay
David Price admits it's going to be tough to pitch against his old pals.
Price said he "made a lot of long-lasting friendships" during his time with the Rays. He said he won't smile when he's on the mound.
The left-hander made his major league debut with Tampa Bay in 2008 and had spent his entire career with the team before the trade.