The Ballydoyle handler left the quartet of runners in this weekend’s 10-furlong Group One at Monday’s confirmation stage, with Kingsbarns, Declaration Of War and Magician also among the entries for the Queen Elizabeth II Stakes on the same card.
However, with ground conditions at Ascot looking certain to be on the easy side, Declaration Of War could be saved for a trip to the Breeders’ Cup meeting in California next month.
Irish 2,000 Guineas winner Magician, not seen since finishing last in the St James’s Palace Stakes at Royal Ascot, also looks likely to miss Champions Day.
O’Brien told At The Races: “Declaration Of War could maybe struggle to run, because if the ground is not good or better he won’t run. If he doesn’t run, he’ll probably go straight to the (Breeders’ Cup) Classic.
“Magician is a little bit the same, he wants real good ground as well.
“Ruler Of The World and Kingsbarns are in and both of those have form with ease in the ground, so maybe they’ll run if the rain keeps coming as it is.
“He’s (Kingsbarns) in the Queen Elizabeth and he’s in the Champion and Ruler Of The World is only in the Champion. Whether the two of them will run in one or they’ll split up, I’m not sure.”
Last year’s Racing Post Trophy hero Kingsbarns has made just one appearance as a three-year-old, finishing tailed off in the Irish Champion Stakes at Leopardstown.
“The Fugue and himself just collided after crossing the road in Leopardstown and we think he knocked his legs together as Joseph (O’Brien) felt he lost his action for a furlong,” said O’Brien.
“He’s been in good form since.”
A total of 15 horses are still in contention for the Champion Stakes, with French ace Cirrus Des Aigles taking top billing.
Godolphin’s hugely talented but fragile five-year-old Farhh is set to make his first appearance since his brilliant win in the Lockinge Stakes at Newbury in May, but connections have yet to decide whether he will run in the Champion Stakes or the QEII.
Jim Bolger has both Parish Hall and Trading Leather engaged at the confirmation stage and Richard Hannon’s Sky Lantern is another with a Champion Stakes entry as an alternative to the QEII.
Mukhadram and Hillstar are others still in the Champion Stakes mix.