Rookie Alfred Morris has been the Redskins' starting running back for all three games so far this year, and he's got 263 yards on 61 carries. There's nothing to indicate the Redskins are unhappy with the job he's done, and no other Redskins running back has had more than two carries in any game since the start of the regular season. The most likely reason Grant is being signed is for depth, as backup running backs Roy Helu (turf toe) and Evan Royster (knee) both got injured in Sunday's loss to the Bengals.
Of course, Grant is a 29-year-old veteran who ran for more than 1,200 yards in each of the 2008 and 2009 seasons for Green Bay. And Redskins coach Mike Shanahan has a history of changing running backs during the season (though, granted, there's usually some sort of injury impetus for him to do so). So it's impossible to completely rule out the idea of Grant starting at running back for the Redskins at some point this season. As of now, they'll bring him in and get him up to speed in their offense, and we'll see what happens down the road.