Watson underwent testing with a specialist on Thursday morning and was due to have further examinations on Thursday night in the hope of being cleared for the decider.
Watson was initially assessed as having category-two symptoms, but that was later upgraded to category-one, triggering an automatic 11-day stand-down from training and playing under the NRL's concussion protocols. The Roosters subsequently asked the NRL to review the category-one grading, seeking a reclassification to category-two, which would remove the mandatory rest period.
One of the country's top neurologists, Mark Parsons, highlighted the risks of rushing Watson back, saying: "I'd be very surprised if an independent neurologist would clear him."
Roosters coach Trent Robinson said the decision was in the hands of professionals and that the club would have an answer by Friday at the latest, when the Roosters are due to hold their main training session. Should Watson be ruled out, it would be his final game for the club before he joins St George Illawarra.