Trainer Andrew Balding is at a loss to explain how the injury which will prevent Side Glance defending his Mackinnon Stakes title occurred.

Fourth in the Cox Plate behind Adelaide at Moonee Valley last Saturday, Side Glance has been withdrawn from Saturday's Group One race with a wrenched fetlock.

Balding said Side Glance was fine when he left the stables on Wednesday evening but the gelding was sore in his box when he returned on Thursday morning.

"He was walking gingerly across his box and there was a bit of filling at the base of his near fore," Balding said.

"With a horse like him we don't want to take any chances so he would have needed to be 100 per cent for us to consider running him."

Saturday's Mackinnon defence was to be Side Glance's third start in Australia this campaign following a third placing behind Fawkner in the Caulfield Stakes on October 11 and his Cox Plate performance.