DeRosa's injury is not yet known.
Moore will make his major league debut sometime in the near future after hitting .286 with a whopping seven home runs and 20 RBIs in 22 games this season for Syracuse.
If you combine the 2010 and 2011 seasons, Moore hit 62 home runs between Single-A Potomac and Double-A Harrisburg. His offensive production continued this season in his first taste of Triple-A ball, and now, he's on his way to the big leagues.
Moore will be able to give manager Davey Johnson a right-handed complement to Adam LaRoche at first base, and he can also play left field. The 25-year-old has minimal game experience in left - he's played just three games there in his career, to be exact - but Moore has been working with outfield coordinator Tony Tarasco recently, improving his fundamentals in the outfield.
He gets the call to the majors just a day after Bryce Harper, and like his former teammate at Syracuse, Moore brings much-needed pop to a Nationals team struggling to score runs.
DeRosa has gotten off to an incredibly slow start to the season, hitting just .081 with three hits in 37 at-bats. The 37-year-old has battled serious wrist issues the last few seasons.
Update: The Nationals have made the move official, recalling Moore and placing DeRosa on the DL with a left oblique strain.