Jackson was carted off the practice field late in Thursday's joint practice with the Los Angeles Rams. He was rolled up on from behind during a live team drill, a few minutes after being involved in a scrap with Rams defensive tackle Aaron Donald.
Jackson was later seen on crutches and wearing a brace on his left knee. The Athletic first reported the extent of Jackson's injury.
"We're concerned, obviously," Raiders coach Jon Gruden said after practice. "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. Hope he's OK."
Jackson, a third-round pick of the Raiders in 2014 who signed a five-year, $56 million extension in 2017, is expected to anchor the right side of the Raiders' rebuilt line. Right tackle Trent Brown signed the richest deal for an offensive lineman in league history at $66 million over six years this offseason.
The Raiders are already thin at guard, with left guard Richie Incognito set to serve an NFL-mandated two-game suspension to begin the season, and Denzelle Good out for at least two months following offseason back surgery.
Jonathan Cooper, who will start in place of Incognito to begin the season, could potentially move to right guard should Jackson be out an extended amount of time.
"We'll have to put our heads together to take a look where at we're going, if we need to," Gruden said.