The team announced the move late Saturday, but it wasn't made official until Sunday.
LaRoche aggravated the injury during Friday's 8-0 loss to the Oakland Athletics and was held out of the final two games of the series. He was the Nationals' leading hitter in several categories and had five home runs and 21 RBIs before getting hurt.
Tyler Moore was called up from Triple-A Syracuse before Sunday's game against the A's to replace LaRoche. Moore batted seventh and went hitless in three at-bats.