Unencumbered will need a new rider in the Golden Slipper with regular jockey Nathan Berry ruled out after being struck down by viral encephalitis.

Berry's manager Ryan Roberts confirmed on Wednesday that the jockey would not be fit to partner the Bjorn Baker-trained youngster who he has ridden in six of his seven starts including a Magic Millions victory.

"He's definitely not going to be ready to ride in the Slipper," Ryan said.

"As for how long he will be in hospital, it's a bit early (to say)."

Berry, who is riding in Singapore on contract, was taken to hospital after he collapsed during a break at Kranji trackwork on Tuesday.

He was later diagnosed with viral encephalitis, a condition that causes inflammation of the brain.

The Singapore Turf Club reported the 23-year-old was likely to remain in hospital for two weeks although Roberts said it was difficult to put a timeframe on Berry's recovery.

"He's had a bad last 24 hours. He's had a raging temperature and they're trying to get that down," he said.

"It attacked his body quite violently."

Berry had planned to return to Sydney to partner Unencumbered in the Todman Stakes at Rosehill on Saturday and has been replaced by his twin brother Tommy.

Tommy Berry said he planned to fly to Singapore on Monday to be with his brother.