The starting right guard was carted off the field Thursday with a left knee injury that he suffered during a joint practice with the Los Angeles Rams.

Oakland Raiders offensive guard Gabe Jackson during NFL football training camp Thursday, Aug. 8, 2019, in Napa, Calif. Both the Oakland Raiders and the Los Angeles Rams held a joint practice before their upcoming preseason game on Saturday. (AP Photo/Eric Risberg)

Los Angeles Rams outside linebacker Clay Matthews (52), left, has a discussion with head coach Sean McVay during the NFL team's joint training camp practice against the Oakland Raiders in Napa, Calif., Thursday, Aug. 8, 2019. (Heidi Fang /Las Vegas Review-Journal) @HeidiFang

Oakland Raiders running back Jalen Richard after the NFL team's joint training camp practice against the Los Angeles Rams in Napa, Calif., Thursday, Aug. 8, 2019. (Heidi Fang /Las Vegas Review-Journal) @HeidiFang

Oakland Raiders quarterback Derek Carr (4) meets and greets members of the Air Force after the NFL team's joint training camp practice against the Los Angeles Rams in Napa, Calif., Thursday, Aug. 8, 2019. (Heidi Fang /Las Vegas Review-Journal) @HeidiFang

NAPA, Calif. — Raiders right guard Gabe Jackson was carted off the practice field Thursday with a left knee injury, delivering the latest blow to a position group that has absorbed its share since the spring.

He is expected to miss about eight weeks, a person familiar with the situation said Thursday night. The development leaves the Raiders without their primary option at either guard position to begin the season. It’s likely the club will explore the trade market in the wake of Thursday’s setback.

Jackson suffered the ailment at a joint practice against the Los Angeles Rams. It occurred early in the morning session when a teammate incidentally rolled up on his leg during an 11-on-11 period, sending Jackson to the grass on the west field at Redwood Middle School.

Hundreds of fans, who were raucous minutes earlier during a player skirmish that forced coaches to cut short a pass-rush period between Raiders offensive linemen and Rams defensive linemen, largely fell silent. Jackson was on the ground about a minute before being helped to the side.

H. Rod Martin, the team’s head athletic trainer, evaluated Jackson. General manager Mike Mayock and quarterback Derek Carr were among the others who checked on him. Before the cart escorted him to a nearby field house, the entire offensive line stood around Jackson in support.

Jackson struggled to place pressure on his left leg.

“We’re concerned, obviously,” Gruden said shortly after practice. “Really concerned about him, but we’ll get more information here when we can be specific for you. I don’t want to speculate.”

Jackson’s absence intensifies the Raiders’ predicament at guard.

In March, they traded left guard Kelechi Osemele to the New York Jets after a season in which he missed more time to injury — five games — than the team thought his medical situation warranted. His departure cleared $10.2 million in cash and cap space.

That left Denzelle Good, re-signed to a one-year contract, to compete for the left guard job. In May, he injured his lower back during the first practice of organized team activities. Mayock said in late July that Good will miss about two months, casting doubt on his Week 1 availability.

Richie Incognito, who signed in late May, is expected to be the Raiders’ starting left guard in 2019, but he will miss the first two games to a suspension for having violated the NFL’s personal conduct policy. The club signed Jonathan Cooper to help mitigate his absence.

With Jackson out, Denver Kirkland worked at first-team right guard. Jordan Devey, predominantly a center in practice this offseason, also saw action there Thursday.

As the Raiders proceed, a trade to upgrade at guard could come.

The market heats up every August when teams overstocked at certain positions acquire a late-round draft pick to deal a player whom they would have otherwise cut. The Raiders, for example, traded safety Shalom Luani to the Seattle Seahawks for a 2019 seventh-round choice on Sept. 1, 2018.

This year, all NFL clubs must reduce their roster size from 90 to 53 players by 1 p.m. PST on Aug. 31.

Jackson demonstrated renowned toughness last season. He played all 855 offensive snaps in the first 13 games despite a partial pectoral tear he suffered in the season opener against the Rams. He also fractured an elbow during the first half of a Week 14 game versus the Pittsburgh Steelers.

The latter injury ended his season.

Thursday’s injury won’t.

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Contact reporter Michael Gehlken at mgehlken@reviewjournal.com. Follow @GehlkenNFL on Twitter.