The injury occurred during the run in last Wednesday's first-up race at Canterbury and was reported to the stewards on Friday following a vet examination.

It's a cruel blow for the Bathurst stable after they had worked tirelessly to help El Mo recover from a abscess in the hoof, bringing him back into racing with hopes of earning selection to the $1.5 million Kosciuszko.

Williams got her wish when El Mo earned the second last spot in the race but the latest injury quickly cut those hopes short, adding to a troubled injury history for one of her stable stars.

"We dropped him straight off to the vet clinic on the way home. They X-rayed him on Thursday and the decision was made on Friday after some discussion," she said.

"He'll have to undergo a knee operation now and spend six months in the paddock. With management we'll see him make a return to racing."

As tough as it would be to do, Williams said any sign of El Mo favouring the injured leg will lead to his retirement.

"The vets are confident, and that's something that you just have to wait and see about," she said.

"He'll tell us, of course, and we've got an open mind about it. At any point if he comes back into work and looks to be favouring that leg again then he would be retired."

Prior to his run at Canterbury El Mo had been backed into a $15 price for The Kosciuszko - the richest race in Australia for country-trained horses.

El Mo's run at Canterbury was his first race start in 10 months and shaped up to be the perfect preparation for the feature event on October 17.

Williams' Healing Hands remains a $51 shot for The Kosciuszko, with two race slots still available, but the Bathurst trainer has previously indicated was isn't targeting the horse at the event.

Gulgong trainer Brett Thompson has Kookabaa - formerly trained by Williams - being the only other horse in the western region as an outside chance of gaining a place in the event.