Ryan Moore has switched to Idaho after US Army Ranger was pulled out of Saturday's Dubai Duty Free Irish Derby at the Curragh.
US Army Ranger produced an unsatisfactory scope yesterday and Moore takes over from Seamie Heffernan who rode the colt to finish third in the Epsom Derby earlier this month.
"US Army Ranger's scope wasn't 100 per cent and unfortunately he won't be running in the Irish Derby," confirmed trainer Aidan O'Brien.
"It's a terrible pity as we were all looking forward to running him."
The Investec Derby runner-up had been promoted to odds-on favourite after doubts surfaced over the participation of his Epsom conqueror Harzand, but the big rematch is now definitely off.
In the updated market, Coral make Harzand and Idaho their 11-8 joint-favourites, with Red Verdon (8-1) and Moonlight Magic (10-1) next in the betting.
"US Army Ranger had been well-backed for the Irish Derby in recent days, so the news he will miss the race will come as a blow to punters who had made him the odds-on favourite," said Coral's David Stevens.
"Connections still have a leading chance on Saturday in the form of Idaho, who now shares favouritism with the easy to back Harzand.
"We await further news on US Army Ranger with interest, as he does hold a Coral-Eclipse entry, and would be a leading contender if lining up at Sandown early next month."
Moore had a winner at Salisbury yesterday courtesy of Wave Reviews for trainer William Haggas.
He also rode The Queen's horse Mainstream but could only finish fifth of six on the 5-2 joint favourite behind Marmajuke Bay.
Ulster trainer Neil Mulholland had a winner at Worcester, Harry Cobden riding Courtlands Prince to victory.