Silvestre de Sousa is long odds-on for the Stobart Jockey Championship after Ryan Moore was advised by a specialist to have "complete rest" having been "a bit sore for the last couple of weeks", reports RacingUK.
Moore has not competitively ridden since Sunday and went on holiday with his family to Portugal soon afterwards.
The star jockey has said he will only return to the saddle when he is fully fit and is now as big as 16-1 with Bet365 to overhaul his Brazilian rival, who is defending his title. De Sousa is now a best-priced 1-4.
De Sousa currently has 59 winners, two clear of Moore and four clear of Godolphin's James Doyle.
"I have been a bit sore for the last couple of weeks and been advised by a specialist that complete rest is the best option," he said in a statement.
"I'm not going to rush it as I want to be 100 per cent for all the major races in the autumn so I'm going to play it by ear, but hope to be back soon."
Moore, 32, would have been expected to ride the Aidan O'Brien-trained Caravaggio, ante-post favourite for next year's 2000 Guineas, in the Keeneland Phoenix Stakes at the Curragh on Sunday.
York's big four-day Ebor Festival begins on August 17, where the likes of Minding, whom the jockey has ridden to five Group One victories, could run in the Juddmonte International Stakes and Darley Yorkshire Oaks.
Speaking at Cork racecourse on Tuesday night, O'Brien said he had been aware Moore "was having trouble with a hip", adding "I knew that the last while and you could see he was lame while he was walking."