"We're concerned, obviously," Raiders head coach Jon Gruden said. "Really concerned about him. But we'll get more information here and we can be specific for you. I don't want to speculate. Unfortunate, but hope he's OK."
The Raiders selected Jackson in the third round of the 2014 NFL Draft out of Mississippi State. The right guard signed a five-year, $56 million contract extension with the franchise in 2017.
Jackson is expected to help anchor the right side of the Raiders' rebuilt offensive line this season. Oakland signed right tackle Trent Brown to the richest contract for an offensive lineman in league history this off-season to protect franchise quarterback Derek Carr.
In Jackson's absence, the Raiders rotated Denver Kirkland and Jordan Devey at right guard for the rest of practice, according to ESPN. Jonathan Cooper could move to right guard if Jackson is out for an extended period.
"We'll have to put our heads together to take a look at where we're going, if we need to," Gruden said.