It's interesting to note that Cooper and Keenan Allen are both available and fit the mold more of posession recievers with seperation. While Cooks may have slowed down some, he's still fast and relies on that speed to get open.
It seems that's Kellen Moore's preference for some reason. There is a current trend in the NFL of getting speedy receivers and stretching the defense over quick 5 yard pass plays that brings the defenses up more to the LOS. Maybe this is another layer of strategy Moore is adding to help put the running game in position to succeed.