A full vet check was in order after the five-year-old pulled up "uneven in his action" after a trial at Randwick on Friday, according to trainer David Hayes.
“It's all a bit of a mystery as X-rays on the knees have come back all-clear,” Hayes told Racenet.
“It may be a deep-seated foot abscess or something like that.
“But we haven't been able to work him for three days, so he is on his way back to Lindsay Park.”
There would be no chances taken with the "very valuable" horse, a course of action backed up by owner Gerry Ryan.
“I’m not going to risk the horse in any way for the sake of winning another race,” Ryan said.
“He might be retired and go to stud. He’s got a nice pedigree.
“He’s a Group One winner. He doesn’t need to win another race.”
As well as forgoing the Group One Doncaster Mile, Fifty Stars is now also in doubt for the Group One Queen Elizabeth Stakes — also at Randwick — on April 11 due to the suspected injury.
“Trainer Mr D Hayes has advised Racing NSW Stewards that Fifty Stars has been found to be lame in the near fore leg and will not accept this morning for The Star Doncaster,” a Racing NSW Stewards statement said on Tuesday.
“He advised the horse will be returned to his Euroa stable whereby it will be assessed by his stable veterinarian.
“He further advised a decision regarding a start in the Longines Queen Elizabeth Stakes will be made once the horse has undergone a veterinary assessment.”
It comes less than a month after the son of Sea The Stars, out of Swizzle Stick, delivered Lindsay Park its 10th Australian Cup thanks to a half-length victory over Regal Power and last year's Melbourne Cup winner Vow and Declare with John Allen on board.