Winona Costin's bid to outride her metropolitan allowance before the end of her apprenticeship has suffered another setback.

Costin has been ruled out of riding at Rosehill on Saturday after being involved in a fall at trackwork on Thursday.

She sustained a cut to her thigh that needed hospital treatment.

Costin remained hopeful she could fulfil her book, but stewards have taken the advice of Racing NSW's Dr David Duckworth, standing down Sydney's 2014-2015 champion junior rider.

Her Rosehill mounts included early favourite Ravi for her bosses Peter and Paul Snowden in the New Caledonia Travel Connection Handicap.

However the Snowden stable says Ravi, as well as stablemates Ryker and Dark Steel, are unlikely runners given the forecast for heavy rain on Saturday.

Costin has only recently returned to riding after she was injured in a race fall at Grafton in April.

She needs 11 more winners to join an exclusive club of apprentices to lose their claim before she comes out of her time at the end of next month.

Hunter Valley apprentice Adam Sewell will replace Costin on Attainment in the Highway Handicap while James Innes Jnr takes over on Grand Condor in the Pacific Boating Handicap.