O'Brien said: "The Duke and Henry are two exceptional horses. It has been a long, hard season for both but they seem in good form at the moment.
"For the Classic you need a horse who definitely gets a mile and a quarter. It is a race where you have to be tough and hard.
"The Duke is an unbelievably tough horse, with a great will to win, and if any horse can cope it will be him.
"We can't be sure that Henry will get a mile and a quarter, and at the moment Johnny is favouring riding Duke Of Marmalade over Henrythenavigator.
"We have booked John Velazquez for whichever one he doesn't ride."