The most anticipated period in John O'Shea's training career is also turning into one of the most testing.

O'Shea, who will replace Peter Snowden as head trainer for Darley Australia in May, lost his second potential autumn carnival star in the space of two days when White Sage broke down on Thursday.

White Sage, an acceptor for the Group One Coolmore Classic at Rosehill on Saturday, went amiss on the training track at Randwick.

The mare, who started her racing career with five-straight wins, landed awkwardly during a routine gallop and suffered a leg fracture.

O'Shea said the four-year-old would be retired.

"It's an unfortunate part of what we do," he said.

"You hold your breath every time you send your good horses out to work along and this time it didn't end up going our way."

White Sage's retirement follows Bernabeu's injury during a piece of fast work on Tuesday morning.

The three-year-old was being put through a special gallop when he faltered at the 200m.

He suffered breaks to sesamoid bones in his off foreleg and remains on the critical list at a Randwick veterinary clinic.