Omaha Beach, the morning-line favorite for Saturday’s Kentucky Derby, will be scratched due to an entrapped epiglottis, trainer Richard Mandella said Wednesday.

The stunning blow to the 145th running of America’s most famous thoroughbred race was discovered when the son of War Front developed a cough after a promising morning workout at Churchill Downs and was scoped, the Hall of Fame trainer said in a statement.

“After training this morning we noticed him cough a few times,” Mandella said. “It caused us to scope him and we found an entrapped epiglottis. We can’t fix it this week so we’ll have to have a procedure done in a few days and probably be out of training for three weeks. We’ll have to figure out a whole new game plan.”

The brawny dark bay colt, who had been assigned 4-1 odds at Tuesday’s draw following wins in the Rebel Stakes and Arkansas Derby, had been slated to break from the No 12 post under Mike Smith, the veteran jockey who rode Justify to the Triple Crown a year ago. Earlier Wednesday, Omaha Beach had galloped one and three quarter miles in preparation for the $3m race, after which Mandella declared the three-year-old as “good as ever”.

Churchill Downs PR (@DerbyMedia) Epiglottis: A triangular-shaped cartilage that lies at the base of the airway just in front of the arytenoid cartilages which cover the airway during swallowing. It is normally located above (dorsal) the soft palate. #KyDerby pic.twitter.com/KyOUGP021r

The scratch opens the door for Bodexpress, son of 2012 Kentucky Derby and Preakness Stakes runner-up Bodemeister, who enters the field for the one-and-a-quarter-mile race in the No 20 slot from the also-eligible list, with every horse from No 13 to No 20 moving over one position in the starting gate.

The revised morning-line odds from track oddsmaker Mike Battaglia issued shortly after Wednesday’s announcement have priced Game Winner as the favorite at 9-2, while Roadster and Improbable are the joint second choices at 5-1. All three are entered by Bob Baffert, who last year became only the second trainer in history to saddle more than one Triple Crown winner when Justify completed the rare sweep of the Derby, Preakness and Belmont Stakes.

The defection of Omaha Beach, who is owned by Rick Porter’s Fox Hill Farm, marks the third time in 11 years the Kentucky Derby morning-line favorite has been scratched after Uncle Mo (2012) and I Want Revenge (2009).