It has been revealed that Alex Johnston is no sure thing of playing in this weekend's preliminary final, despite being named on Tuesday afternoon.

Meanwhile, centre Campbell Graham is also playing through a painful rib injury and is requiring pain killing injections to make it through the finals.

Johnston failed to finish last weekend's winning semi-final effort over the Cronulla Sharks, with the NRL's top try-scorer coming from the ground shortly after halftime.

A hip flexor injury was revealed to be the reason, and the club have stated that he will need to pass a fitness test to take his spot in the line up this weekend.

Johnston himself admitted it would be "touch and go."

"I'll be doing everything I can to get out there," Johnston said on Tuesday.

"I'll start running hopefully towards the end of the week.

"I'm not a doctor but I don't think you can needle it, it's a muscle sort of thing so it's physio, rest and ice up.

"It will be touch and go. I feel pretty confident the way it's recovered from yesterday to today. If it (stays on that) trajectory, I'll be right."

Should the winger be out, Richard Kennar or Josh Mansour would come into the team, with Jaxson Paulo missing the 22-man squad.

Graham, on the other hand, is said to need about a two to three week recovery to be back at full fitness, but will play through the injury until the finals campaign comes to an end.

In better news for the Rabbitohs, who have also lost Siliva Havili for Saturday's game with Penrith, Jai Arrow has confirmed he is fit and will be on deck for the grand final qualifier.

Arrow was taken from the field with a groin twinge during Saturday's win over Cronulla, but AAP have confirmed his scan has cleared him of any damage.

South Sydney will play Penrith on Saturday evening for a spot in the grand final.