Proisir, second favourite to win next month's Doncaster Mile, will miss the rest of the Sydney autumn carnival and his promising racing career is in doubt.

Considered a Group One winner-in-waiting, the three-year-old has damaged a tendon.

The injury is expected to sideline Proisir for six months but trainer Gai Waterhouse said there were few guarantees he would return to the racetrack.

"He's injured his tendon so he's out for the autumn," Waterhouse said.

"I will speak with the owners to see whether they want to spell him and bring him back or retire him to stud."

Proisir made a rapid early-season rise to the top of Australian racing, winning his first race start at Hawkesbury on August 1 before contesting the 2012 Cox Plate less than three months later.

In between he won the Spring Stakes at Newcastle by almost six lengths and figured in a thrilling Spring Champion Stakes duel with It's A Dundeel when second in the Randwick Group One.

In two runs this campaign, Proisir was placed in the Hobartville Stakes and ran second again to It's A Dundeel in the Randwick Guineas.

Proisir was expected to head towards the Doncaster Mile, in which he was a $7 chance, via Saturday's Ajax Stakes at Rosehill but he was a notable omission at pay-up time.

In spite of Proisir's defection, pre-post Doncaster operators still believe Gai Waterhouse will train her eighth winner of the famous mile.

Glamour colt Pierro is into $2.50 after his Canterbury Stakes win over stablemate More Joyous who is on the second line of betting at $11.