I think Washington will miss Helu's speed and pass-catching out of the backfield. Last season, the offense was unproductive until they found some success with Helu running the toss stretch. When that play got on film, defenses had to adjust, which created openings for other ways to attack.
None of the backs on the roster have Helu's burst. Alfred Morris is strong and has good agility, but unexceptional straight line speed. Evan Royster is faster than Morris, but not a burner. Grant was never a speed-demon -- he's more in the Morris mold.