Something to notice is that Harris is only playing 31.9 minutes on the year (I guess Johnson has a major crush on Jordan Farmar).
If you adjust Harris' minutes to match D-Will's, here is how their stats match up on the season:
Williams
21.3 points, 9.7 assists, 3.9 rebounds, 1.2 steals, 3.5 turnovers
Harris
17.4 points, 8.8 assists, 2.8 rebounds, 1.3 steals, 3.4 turnovers
I'm not going to be an idiot and say he will perfectly replace D-Will, but I'm hoping the drop-off isn't as drastic as some on this board would like to think.