Blow for Rabbitohs as Roberts ruled out with thumb injury

South Sydney have been dealt a blow on the eve of their preliminary final against Canberra, with representative star James Roberts ruled out due to the dislocated thumb he suffered against Manly.

Roberts trained away from the rest of the squad on Thursday and did not complete any ballwork or contact work with coach Wayne Bennett confirming the worst soon after.

Bennett said he did not expect Roberts to miss the grand final should the side make it, but could not expect him to get through the Raiders.

"We've managed inconvenience all season so it's no big deal," Bennett said.

"We lost one of the best players in the game (Greg Inglis) before the season even started, or after two games, and from that point on we've had lots of injuries or lots of players missing and we've managed to get the job done and I have no doubt these guys will tomorrow night."

Roberts' omission sees Campbell Graham start in the centres and Corey Allen will come onto the wing.

Graham showed his versatility last weekend when his defensive work out wide helped blunt Manly's attack as the Rabbitohs prevailed 34-26 to book their ticket to Canberra.

Bennett conceded around a third of his side are carrying injuries into the grand final qualifier.

"Probably 10, those guys are fit enough to play, they've been managing, it's just that time of the year, he said.

"You look at the Raiders and the other clubs, they'd have the same problems, they're manageable and doctors and physios are great. We wouldn't play them if they're not fit enough to play.

"I think this year is the worst year they've had for injuries over the last four years.

"Key players in the club not playing, at times we've played with 40 per cent of our salary cap.

"It's been remarkable to do what they've done, it's a credit to them."

After leaking 56 points in the last two weeks, Bennett could be forgiven for having sleepless nights about the threats posed by the likes of Wighton, Leilua, Rapana and Nicoll-Klokstad.

"We've defended well for periods this season, we didn’t defend the week before that, so we’ve had two pretty ordinary weeks of defence," Bennett said.

"But we managed to score enough points to win the game and obviously we go back tomorrow night and try and get our defence right and I'm confident we will.

"Last week were our individuals rather than the system that let us down."