I think Grant is likely to play a big role for the next few years, barring injury. At 28, he has at least a couple more good years left, and his work ethic has become legendary among his teammates. I also think that, unless Starks clearly shows that he is the better player, Grant will get his starting job back next year. He's under contract, and the Packers would be wise to use him while they can. There's no need to rush Starks unless he is just too good to keep off the field.
I don't see how the loss of Grant has made the Packers better. The offense, particularly the running game, clearly struggled without him. The O-line did not open better holes for Jackson, as far as I could tell. It looked more like the linemen lost confidence and regressed without Grant running the ball. The offense is now to the point where it is about as good as last year, but it is not better than last year. If the Packers are improved, it is because the defense is improved, and to a lesser extent, the special teams.