The Rabbitohs copped a major blow when fullback Alex Johnston went down with a concerning hamstring injury in Friday's 18-14 loss to the Sydney Roosters.

Not only did Anthony Seibold's side surrender top spot, Johnston is facing the prospect of several weeks on the sidelines and will undergo scans on Saturday.

The fullback lasted just two-and-a-half minutes before coming off, forcing a reshuffle which saw Dane Gagai move to fullback and Tyrell Fuimaono into the backline.

Seibold said it wasn't clear how long Johnston will be missing, but it had been assumed that Inglis would seamlessly slot into the No.1 jumper against Brisbane next week.

Inglis is scheduled to return from a broken thumb against the Broncos - having been missing since before State of Origin III.

However Seibold revealed he's uncertain about Inglis' return date and was wary about risking a recurrence of his injury just three weeks out from the finals.

"We are hopeful Greg is back next week," Seibold said.

"He had his visit to the surgeon yesterday and he may miss another week.

"I am not going to risk him.

"We have got ourselves in a really strong position and if the surgeon thinks he needs another week that is what we will do."

Gagai battled admirably after being shifted to fullback however he hadn't trained there all season and the Rabbitohs sorely missed Johnston.

"It's his hamstring so until he gets a scan we have no idea how bad it is," Seibold said.

"He has got a history of hamstring injuries so we just need to make sure we get a scan tomorrow morning and get it assessed by our medical staff.

"Obviously he is not going to be alright next weekend. You don't come off the field in the first minute and turn around to play six days later."

Johnston's injury means almost all of the Rabbitoh's starting outside backs are now sidelined.

Winger Robert Jennings (hamstring) is racing to be back before the finals while fellow flanker Campbell Graham (wrist) missed Friday's loss to the Roosters.

Utility Adam Doueihi (knee) has also been ruled out for the season.

"We are under the pump a bit with our outside backs with Greg Inglis and Robert Jennings and Campbell Graham all injured," Seibold said.

"Now Alex. A few of those guys were performing really well for us. Adam Doueihi is included, he is gone for the year.

"I was really happy with our depth tonight."

Australian Associated Press