Champion rider Michael McNab will be sidelined by injury for an indefinite period.

After his first ride at Tuesday’s trial meeting on the synthetic track at Cambridge, the two-time National Jockeys’ Premiership winner was unable to fulfil his remaining commitments due to severe back pain.

“He had his first ride and afterwards he was in agony and obviously couldn’t carry on,” McNab’s agent Aidan Rodley said.

“He went to pack up his gear and couldn’t even do that, (trainer) Colm Murray had to help him.”

McNab was taken to hospital and X-rays revealed a fractured vertebra, which is thought to stem back to a fall on June 15 at Te Rapa where he parted company in the straight with the Stephen Marsh-trained favourite Takeshi.

Following the incident, apprentice Ngakau Hailey pleaded guilty to a charge of careless riding and was handed a month’s suspension.

At that stage, McNab was in the running to retain his premiership crown, but in his absence after the fall Warren Kennedy gained a break in the chase for the title.

“We thought Michael was on track to be winning his third premiership in a row at the time of the fall,” Rodley said.

“He had twice come from a long, long way behind. He was 15 behind and 19 behind earlier on to come back and square up with Warren.”

McNab has nevertheless enjoyed another terrific season with 124 winners and five at Group or Listed level, with his mounts earning more than $4 million in prizemoney.

“I would say a highlight would be riding a stakes winner (Maharba, Listed Hareeba Stakes, 1200m) for Grahame Begg at Mornington,” Rodley said.

“That’s the second year in a row that Michael has gone to Australia and won a stakes race.

“He’s getting very few opportunities over there, but he’s showed he’s up with the best in Australasia.”

Last season, McNab combined with the Barry Lockwood-trained Petronius to claim top honours the Listed Lightning Handicap (1000m) at Eagle Farm.