Jim Cassidy is paying a price for his historic 100th Group One triumph with the champion jockey in serious doubt for Saturday's Rosehill meeting.

Cassidy stood himself down after one ride at Warwick Farm on Wednesday with a rib cartilage injury courtesy of an over-the-top celebratory hug from one of the owners of Zoustar after the colt's Coolmore Stud Stakes win.

The Flemington victory brought up a century of Group One winners for Cassidy and gave a syndicate of owners, headed by Sheriff Iskander, an $18 million windfall with the three-year-old's sale to Widden Stud.

"I weighed in and Sheriff gave me the biggest bear hug," Cassidy said.

"I have been icing it on and off but it's been getting worse and that ride just put me over the top."

Chief steward Ray Murrihy said Cassidy also rode in extreme discomfort at Randwick last Saturday.

"He's been managing it for a few days but he aggravated it today and they tell me it's pretty painful," Murrihy said.

"One would anticipate it's not something you could resolve immediately ... he's got to get it right."

With Cassidy booked to ride at Canterbury on Friday, Murrihy said he expected a clearer indication of Cassidy's fitness on the morning of the night meeting.

Cassidy has five Rosehill rides, four of them for Chris Waller.

His mounts for the premier trainer include Specific Choice, certain to be one of the early favourites in the Cellarbrations Tamworth Superstore Handicap (1500m).

A winner of two races from his six starts, Specific Choice was considered one of the unlucky runners at Rosehill on November 2 when he finished off strongly between horses to be beaten a length into fourth place behind Corryvreckan.

It was only his second run in 12 months and his effort to finish runner-up in a photo finish to this season's Group One winner Rebel Dane in a Rosehill race in October last year has stamped him as a four-year-old of genuine potential.