The Redskins signed Johnson to a contract Wednesday.
With Alex Smith (lower leg) out for the season and Colt McCoy (lower leg) likely facing a similar fate, Johnson will step in as the backup to Mark Sanchez ahead of Washington's game Sunday against the Giants. The 32-year-old last threw a pass in an NFL regular-season game in 2011, but his familiarity with Washington's offensive scheme from having previously worked with coach Jay Gruden should help him get up to speed quickly.