Days out from the semi-final of the NSW Breeders Challenge, the unbeaten two-year-old has been withdrawn from the group one series with a hoof complaint.

Despite winning his heat at Young last month, making it a perfect six from six, subsequent work revealed a quarter crack.

The Temora horseman attempted to alleviate the problem before deciding to pull the pin on the campaign.

"I knew he wasn't 100 per cent at Young as he hung that bad going around that last corner and it's not like him," Walker said. "I knew working him up to it he just wasn't quite right somewhere but you can't see a quarter crack so you just don't know.

"I said we would qualify and worry about it later but I noticed a little blood on his back foot on the jogger and thought he must have kicked something and then the next day I was bowling along steady and it opened up a bit.

"We tried to prop him up for the race but the horse had to come first."

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Walker is disappointed not to be able to add to his first group one race won with Mister Rea in March.

Especially as Mister Rea is the fastest two-year-old in Australia this season.

"It's just a bit of bad timing but he still has all life ahead of him," Walker said.

"It's better than having a broken pedal bone or a bowed tendon and going forward, while they do seem to recur, there are plenty of times where he won't have it and he will win plenty of races."

It means Walker will miss the expected showdown with son-in-law Jarrod Alchin.

The former Temora horseman now leads the charge with another unbeaten group one winner, Our Ultimate Byron.

Mister Rea won't race again until his three-year-old season, with Walker eyeing off the NSW Derby in February ahead of another trip to Bathurst.

"I'll get him back going again when he's right and he's not that highly assessed so he will have a couple of runs in the Riverina and if he's going good enough I'll have a go at the Derby when the Derby comes," he said.

Fields for the semi-finals will be released on Monday.

Mister Rea's withdrawal paves the way for Eucan Roll to sneak into the semi-finals.

After finishing second behind Mister Rea at Young, trainer David Micallef elected to bypass the second round of heats at Wagga on Friday.