Society Rock has met with a setback and is unable to defend his crown in the Betfred Sprint Cup at Haydock on Saturday.
The James Fanshawe-trained six-year-old, who beat Gordon Lord Byron last September, suffered a bang to the head on Sunday.
"Society Rock had a fall (on Sunday afternoon) and suffered a bang to his head," read a statement posted on www.jamesfanshawe.com.
"He has some concussion and is receiving medical treatment from the vets at Newmarket Equine Hospital.
"He remains at Pegasus Stables, is bright, standing up and eating.
"We will continue to receive medical treatment and will require careful management over the next few days."
Society Rock showed he had lost none of his spark when making a successful seasonal debut in the Duke of York Stakes on the Knavesmire in May.
He has since finished second to Lethal Force, whom he beat at York, in both the Diamond Jubilee Stakes and the Darley July Cup.
Lethal Force heads 18 horses left in the six-furlong showpiece at the confirmation stage.
Clive Cox's speedster will attempt to regain winning ways after being beaten into second place by Moonlight Cloud in the Prix Maurice de Gheest at Deauville four weeks ago.
Cox has a second string to his bow in Reckless Abandon, who was last seen finishing fifth in the King's Stakes at Royal Ascot and who has yet to reach the heights of his juvenile campaign.
Last year's runner-up Gordon Lord Byron tries again for County Tipperary-based handler Tom Hogan.
The other Irish-trained possibles are Edward Lynam's duo of Slade Power and Viztoria, plus Cristoforo Colombo from Aidan O'Brien's stable.
William Haggas has two interesting contenders in Stewards' Cup hero Rex Imperator and Group Three scorer Heeraat, while Jim Goldie relies upon Hawkeyethenoo after taking out Jack Dexter.
South African trainer Mike de Kock could look to Kavanagh to give him sprint honours on these shores after going close with Shea Shea this summer.
Among others in the mix are the Richard Fahey-trained Garswood, winner of the Group Two Lennox Stakes at Glorious Goodwood, John Gosden's Swiss Spirit and Robert Cowell's stalwart Kingsgate Native.