Per the Des Moines Register.
It sounds like the hamstring injury that's been bothering him all offseason is continuing to do so. It's significant if he misses a chunk of time, as I really think he will develop into our go-to-guy, provided he can stay healthy. From the article, it sounds like Darks isn't healthy yet, and when he does get healthy, he has quite a bit of work to do. Hopefully he wouldn't miss too much time when healthy, and perhaps they would just phase him in as the 3rd or 4th WR until he gets back in shape and a complete feel for the offense.
Fortunatley for ISU, receiver is an area where it seems like we have bodies. I am confident that between Hamilton, Jones, Reynolds, Johnson, and whomever else we throw out there will be able to step in. I just hope that Darks can get back soon, because we will definitley need him if we want to have a successful season.