The 22-year-old pulled up short when scoring the best of his three tries late on at ANZ Stadium, and will have scans this weekend to determine the extent of the injury.

Should he miss the final two matches of the season he might still have done enough to secure the NRL's leading try scorer crown - he now has 22 from as many games, four clear of his closest challenger Suliasi Vunivalu.

"He's probably got something there so we'll have to assess that, which is really unfortunate because he's been playing some great footy and looking at records in and around the club scoring tries," Souths coach Michael Maguire said.

"We'll have to get a scan on that one but he's a player that's really improved at fullback at this back end of the season. I think he's equal to Nathan Merritt at the moment so he'll work hard to get back and have another crack at that."

Johnston might also have done enough this season to make the number one jersey his own next season, which would force returning superstar Greg Inglis to play in the centres.

He saved his best for last on Friday, carving his way through the Warriors' porous defence 40m out from the try line before executing a superb in and away on Roger Tuivasa-Sheck to score in the corner.

His heroics came in front of just 6213 fans at ANZ Stadium, the second lowest NRL crowd of the season. Only Manly's win over Newcastle in round 14 attracted fewer fans, and that was in teeming rain at Lottoland.

Those who did turn up might well have witnessed Kieran Foran's last game for the Warriors ahead of his impending move to Canterbury next season. He copped a heavy knock to his back early in the match and stoutly stayed on the field until the 74th minute.

They also witnessed Jacob Lillyman's last game of the season after he was forced off with a torn bicep.

South Sydney suffered an early blow when captain Sam Burgess was ruled out with a rib injury suffered last week, and they looked rudderless early on without their skipper.

Maguire would have delivered a stern message at half-time, telling his charges to lift after a lethargic opening. Whatever he said, it did the trick and he's now nursing a third-straight late-season win.

"We've gone from averaging around 65 per cent completions throughout the season to getting up above 80," Maguire said.

"If you want to be a successful team, that's what you need to do. When you do that you give yourself an opportunity and now we're starting to see that off the back of our big men running forward and building our game."

The sparse smattering of supporters reflected the energy and enthusiasm being displayed on the field in this late-season fixture during a poor-quality first half.

This was a rare sighting of the Warriors in Sydney - they hadn't played here since round 13 loss to Parramatta at the same venue. And there was to be no joy for them again on this occasion as the club slumped to a seventh straight defeat.

"We've just got some individuals that aren't quite ready, that's the reality of where we are at," Warriors coach Stephen Kearney said.

"It's been a little bit the same same the last couple of weeks, that's where we're at as a footy team. Its pretty challenging, there's no doubt about that but we ve got to keep forging forward.

"I don't know if inexperience is the right word for it. There was some good patches, but there were some good patches last week against the Raiders. But we fell away at certain parts, there were some individuals who just aren't ready for this level."

Neither side showed a huge amount of application in defence, and the referees weren't in the mood to let it flow either.

Despite entering this one in a recent purple patch of form, Souths were the flatter of the two sides in the opening exchanges and allowed the Warriors to carve out an early lead through Tuivasa-Sheck just six minutes in.

He finished off a backline move moments after a lack of communication between Bryson Goodwin and Johnston led to the latter spilling a simple kick on his own line.

Tuivasa-Sheck looked dangerous every time he touched the ball and Souths struggled to get a foothold until they earned a penalty and had a shot at the Warriors line, which turned out to be decidedly flimsy.

It was the Rabbitohs turn to spread it wide and Johnston touched down for the 20th time this season, running through a gaping hole between Solomone Kata and Ken Maumalo.

Simon Mannering put the visitors back in front when he powered over under the posts from close range on the back of three straight penalties, grounding the ball sufficiently enough for the Bunker.

Yet South Sydney managed to go into the break level despite being on the back foot all half.

Young gun Cameron Murray scored his first NRL try when he charged through some non-existent defence, before Adam Reynolds slotted a late penalty goal.