Cooper Cronk's race against the clock to be fit for the NRL grand final has begun after scans confirmed a serious left shoulder injury suffered in Saturday night's win over South Sydney.

The Roosters halfback went for scans immediately after the game and his club announced on Sunday morning they showed a severe left rotator cuff injury.

Rehabilitation has already begun on the injury and the Roosters said in a statement that the superstar halfback would be given until kick off next Sunday to prove his fitness.

Former Queensland and Australian playmaker Cronk was brought to Bondi Junction to deliver the premiership that Mitchell Pearce couldn't and now he's in doubt to face his former club Melbourne in the season decider.

Cronk was injured after being crunched by Adam Reynolds as he attempted to break a 4-4 deadlock and got up clutching his left shoulder.

After the Roosters physios worked on him at half-time, he was visibly hobbled after coming back out and could barely raise his arm.

The half-back was little more than a passenger at times in the second half, only putting his body in front of opposition ball carriers, hoping his teammates would get there to help out.

But in front of a record crowd of 44,380 in the final game at the Sydney Football Stadium before it's redeveloped, the minor premiers won through to their second grand final in six years.

In the end, the Roosters' goal-line defence proved the difference, with Souths enjoying 30 tackles inside the opposition red zone but unable to cross for a try.

The Roosters went ahead 8-4 when Joseph Manu, despite the attention of Greg Inglis and Alex Johnston, popped a beautiful pass in traffic to send Blake Ferguson over.

The Rabbitohs were clearly targeting Cronk, with Junior Tatola smashing him after he had passed the ball.

However, Sio Siua Taukeiaho couldn't convert the two points from the resulting penalty.

From the next set the Roosters iced the game after centre Paul Momirovski, who was a surprise late inclusion for the suspended Latrell Mitchell, crossed for the decisive try.

When Luke Keary lofted a bomb, Campbell Graham failed to reel it in and Momirovski, who was playing the second game of his career, pounced.

The Roosters will receive a big boost going into the grand final, with Mitchell and prop Dylan Napa to return.

Roosters skipper Jake Friend will be free to play next week, despite being charged with a grade one dangerous contact charge for a tipping tackle on Greg Inglis in the first half.

Friend will likely enter an early guilty plea and receive 75 points, under the threshold for a ban.

AAP