The NSW Waratahs seem set to thrust Super Rugby rookie Mack Mason into the five-eighth hot seat in Sunday's showdown with the table-topping Crusaders in Sydney.

With Bernard Foley in extreme doubt, as he continues to battle post-concussion symptoms, 21-year-old Mason is the frontrunner to replace the Wallabies playmaker.

Coach Daryl Gibson won't name his team until Friday, but winger Andrew Kellaway appeared to let the cat out of the bag on Thursday when he backed the little-known Mason to have doubters "eating their words".