ADELAIDE ruckman Sam Jacobs has been ruled out of Thursday night's clash with Geelong with a knee injury.

The Crows haven't set a timeframe for his return, with back-up big man Reilly O'Brien set to play his first game at AFL level since 2016.

Jacobs sustained a knock to his knee in last Friday night's 26-point win against Sydney at the SCG.

He was assessed before returning to finish the game.

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The durable 30-year-old, who has played 198 games for the Crows, trained with the rehab group at Adelaide Oval on Tuesday morning.

"(Jacobs) has some long-standing issues in that knee that were exasperated by a collision injury he had against Sydney on the weekend," Crows general manager of high performance Matt Hass told

"We obviously have to respect that. He's an older player and there's wear and tear in there.

"Unfortunately, this week he hasn't quite done enough to play against the Cats and it will be a little bit of a week-by-week proposition moving forward.

"We'll have to reassess that midweek, make sure we're comfortable with how he's tracking before we provide any clearer timeframes on his return."

Jacobs' injury opens the door for O'Brien to get a rare opportunity in the senior side.

Selected by the Crows with the ninth pick in the 2015 rookie draft, O'Brien's only two AFL games came in round 20-21 of the 2016 season when he filled in for an injured Jacobs.

O'Brien missed most of last season with a shoulder reconstruction, but trained strongly during the pre-season and had an encouraging hitout in the Crows' round one SANFL win against Sturt last Saturday.

Reilly O'Brien is tackled by Scott Lycett during the JLT. Picture: AFL Photos

Crows midfielder David Mackay said the team had confidence in O'Brien to step up to the level.

"He still moves really well like Sauce, he's got great follow up on the ground and he's an elite runner for a guy his size," Mackay said.

"If he does come in, it's a great opportunity for him.

"He's done a lot of work and really challenged Sauce in that role in the pre-season, so if that's the way they decide to go, he's more than capable."

The Crows also have rookie-listed ruckman Paul Hunter at their disposal.

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