ADELAIDE ruckman Reilly O'Brien is prepared to bide his time behind Sam Jacobs, but is also willing to play up forward if the Crows want to carry two big men into the finals. 

O'Brien made an impressive debut in last Saturday night's 138-point belting of the Brisbane Lions at Adelaide Oval, having 29 hit-outs, 16 possessions and kicking a goal.

The 21-year-old was elevated off the rookie list and came into the side to replace Jacobs, who was sidelined with an ankle injury. 

Jacobs will be assessed this week before a decision is made whether he will return for Sunday's away clash with Fremantle at Domain Stadium.

"I'd love to continue to play up there and play as many games as I can, but in saying that, I'm happy, when 'Sauce' comes back, to go back to the SANFL and to be the back-up,'' O'Brien said.

"Not sure (if Jacobs will come back) at the moment.

"Hopefully he is, but if he's not, I'm ready to go.''

The Crows could decide to play O'Brien and Jacobs in the same side.

''I'll continue to work on my forward craft as well as my ruck craft,'' O'Brien said.

''It's just a matter of what the coaches want with the team balance.

''With 'JJ ' (Josh Jenkins) in the team, who plays that tall forward role, it makes it a little bit hard.

"But in saying that, I'll keep playing forward, and I do a little bit of that in the SANFL.''

O'Brien played the majority of his junior football as a defender and it wasn't until he joined the Crows in the 2014 rookie draft that he focused on becoming a ruckman.

He's worked closely with Crows assistant coach Matthew Clarke and Jacobs in developing his skills over the past two seasons. 

''I've spent a heap of time with him, going through vision and I wasn't really a ruckman before I came here,'' O'Brien said. 

''I've learnt my whole ruck craft from Matty Clarke and Sauce.

''My back days are behind me.''

Half-back flanker Paul Seedsman will also have a fitness test this week after being forced out of the win over the Lions in the first quarter with a corked buttock.