Forecasting a defensive impact player as a rookie before the draft is premature. My defensive impact player for the 2008 season wasn't the DROY. The kid whom I think of as an impact player was Phillip Merling of Miami. He played a key role as a 3-4 DE in helping turn a 1-15 team into a division champion. I didn't see much of Arizona, so I don't know what DRC did, but you'd have to look at the NFC Champions to see how he helped them. I'd give the two rookie CBs for Kansas City some love, for all that team's Herminating, they were a scrappy bunch who kept games close.
Raji is going to have his hands full with NFL OC/OG, Wilfork wasn't an impact player as a rookie, I'm not really expecting Raji to set the world on fire either. Linebackers tend to get noticed more and are more likely to wind up a DROY with their stats, Curry is the most likely guy to use his speed and smarts to run down a lot of RBs and get the big numbers, yet I'd hesitate to call him the impact player.
A good question to ask after the draft once we see what prospects are drafted into what situation.