ESPN first reported the visit.
Raiders coach Jon Gruden described the situation as "unfortunate" and blamed the 2011 collective bargaining agreement that restricts the number of offseason workout programs and the intensity at the workouts.
"Everybody is an expert out there on Hackenberg and thinks he can't play," Gruden told the media after the third and final day of Oakland's minicamp, per ESPN.com. "It's unfortunate, this whole collective [bargaining agreement]. How do you develop a quarterback? I don't know how you do it.
"[Hackenberg] has been working on changing his stroke, his passing motion, and I think he did that. We just didn't have enough reps to take a good look at him. Since we were further along the road with some of our other guys, we didn't have the space."