Raiders HC Jon Gruden told reporters that they had some “concern” about Jackson’s injury after he couldn’t put any pressure on one of his legs before being taken off on a cart.
Oakland could turn to either Denver Kirkland or Jordan Devey in place Jackson to open the season.
Jackson, 28, is a former third-round pick of the Raiders back in 2014. He was in the final year of his four-year, $2.952 million contract when he agreed to a new five-year, $56 million extension that included $26 million guaranteed with the Raiders in 2017.
Jackson stands to make base salaries of $6.75 million and $9.35 million over the next two years of the agreement.
In 2018, Jackson appeared in 13 games for the Raiders.