Sydney Roosters star Cooper Cronk is battling a "severe left rotator cuff injury" but will be given until the last minute to prove his fitness for the NRL Grand Final.

Cronk suffered the injury during the Roosters' 12-4 win over the South Sydney Rabbitohs in their Preliminary Final on Saturday.

The playmaker is now in danger of missing the decider against his former club, Melbourne Storm, on September 30.

"Cooper Cronk was taken for scans at full-time after sustaining a shoulder injury during the first half of the match," the Roosters said in a statement on Sunday.

"Scans have revealed that Cronk sustained a severe left rotator cuff injury.

"Rehabilitation on the injury has commenced and he will be given until kick-off to prove his fitness."

Cronk's absence would be a blow for the Roosters, while the Storm may be without star full-back Billy Slater, who is contesting a ban for a shoulder charge.