The starting right guard is set to miss at least six weeks with a knee injury, NFL Network's Tom Pelissero reported, per a source informed of the injury.
Vic Tafur of The Athletic first reported Jackson would miss time into the regular season.
Jackson was carted off the practice field Thursday after getting rolled up on. Coach Jon Gruden said he was "concerned, obviously" about Jackson's injury but didn't speculate further.
It might be a few weeks into the 2019 campaign until Derek Carr has his full complement of blockers in place.