It's a very minor injury. 2-3 weeks most likely. Stoops isn't dumb enough to push his best player when he's dinged up and frankly does not need any sort of preseason to dominate.
Anyone know the status of Gronk's injury? Best I can find is that he is "week-to-week." Perhaps it would be a good idea to hold him out against NAU so he is 100% going against Iowa.
Barring a setback he'll an in all likelihood play against Iowa. No need to rush him back against CMU and NAU. I heard he probably would have played if these games were vital.
Waiting for a back injury to heal is like watching grass grow. It will come around when it comes around. There's no real diagnosis for it unless something's torn or broken.