Champion jockey Zac Purton will be declared this morning for rides at Happy Valley on Wednesday night despite disturbing scenes when he collapsed dismounting from Yeung Sing after race four and was rushed to hospital.

Purton had difficulty riding out Yeung Sing then returned to the unsaddling area distressed and needed assistance to dismount.

Once out of the saddle, Purton was unable to stand up and had to be stretchered away by club medical officers then taken to nearby Union Hospital.

A scan later showed Purton had a bulging disc in his back that was pressing on a nerve but he remained hopeful that he would be able to ride on Wednesday night. Chief steward Kim Kelly said Purton would be declared for runners but with stand-by jockeys nominated.

"I've had a problem for a long time with a nerve in my back and when it flares up like that, my legs won't support my weight and I can't stand. It's something I need to constantly manage and that normally keeps it at bay," Purton said last night. "The irony of this happening today is that I was due to go in and see my chiropractor Monday morning for some maintenance on it but it has sneaked up on me."

Purton was in good spirits and explained the issue was something that could be gone just as quick as it arrived.

Fortunately his mood wasn't darkened by having missed anything at Sha Tin in his absence.

Douglas Whyte took over on fifth race favourite Sensation, which had little luck in the straight before his eighth to Multimax, and on Tonyboy, who was trapped wide from his outside draw and ran unplaced.

Brett Prebble was Purton's replacement on Domineer in race seven, who led and finished last, while Matthew Chadwick took the reins on Numero Uno and also led before running unplaced.