go on packers.com and listen to the interviews and read the coaches transcripts this is clearly stated.... He was not hit in practice and he only played the last pre-season game one play...
One more article you may want to read:
http://www.jsonline.com/sports/packers/36513984.html
If your lazy I copied and pasted the important parts. below
Motivation to run: Besides a victory Monday night, there is incentive for running back Ryan Grant to finish up strong.
The contract Grant signed in August contains some valuable incentive earnings, which Grant is approaching. If he has a solid finish to his year, he could wind up pocketing $1.5 million on top of his $4.25 million salary.
The Packers wanted Grant's contract to be incentive-based and he agreed, so a good deal of the money must be earned through performance.
By topping the 1,000-yard mark, Grant earned a $500,000 escalator that will be tacked on to his 2009 earnings. That figure will jump to $1.5 million if he reaches 1,250 yards, which would require him to gain 214 yards in the final two games.
Entering Saturday, Grant ranks sixth in the NFC in rushing yards with 1,036. If he remains in the top 10, he will receive a $150,000 bonus. If he finishes in the top five, that number jumps to $500,000.
Grant's four-year deal is worth $20 million, but it also includes an additional $10 million in incentives, escalators and performance bonuses.