The news was less positive for utility forward Siliva Havili (calf), who coach Jason Demetriou said wouldn't be named in their team list on Tuesday afternoon.
Havili, Johnston and Arrow (groin) all failed to train at Redfern Oval on Tuesday after the Rabbitohs claimed a 38-12 win over Cronulla to secure their fifth-straight preliminary final berth.
Johnston has a hip flexor issue and faces a late fitness test to be cleared to play the Panthers.
"I'll be doing everything I can to get out there," Johnston said.
"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."
If Johnston fails to overcome the injury, Josh Mansour, Jaxson Paulo and Richie Kennar are contenders to replace him.
Arrow has been battling his groin issue since first hurting it against Penrith in round 23, before aggravating it to its most painful point on Saturday night.
But said he was not under any doubt for this weekend's preliminary final.
"I got a scan and it's pulled up all sweet," Arrow told AAP.
"It's just an ongoing thing that has been an issue for a couple of weeks now. I'll manage it through this week and be right to go Saturday."
The Rabbitohs remain without banned prop Tom Burgess, while Penrith won't be able to call on prolific winger Taylan May as a result of a suspension for a high shot in their qualifying final win over Parramatta.
Charlie Staines is the likely contender to come in for May.
The Eels, meanwhile, will give centre Tom Opacic until kick-off against North Queensland on Friday to overcome a hamstring injury.
